8. Storage Configuration

The Storage section of the graphical interface allows you to configure the following:

  • Periodic Snapshot Tasks: used to schedule the automatic creation of ZFS snapshots.
  • Replication Tasks: used to schedule the replication of snapshots over an encrypted connection.
  • Volumes: used to create and manage storage volumes.
  • ZFS Scrubs: used to schedule ZFS scrubs as part of ongoing disk maintenance.

These configurations are described in more detail in this section.

8.1. Periodic Snapshot Tasks

A periodic snapshot task allows you to schedule the creation of read-only versions of ZFS volumes and datasets at a given point in time. Snapshots can be created quickly and, if little data changes, new snapshots take up very little space. For example, a snapshot where no files have changed takes 0 MB of storage, but as you make changes to files, the snapshot size changes to reflect the size of the changes.

Snapshots provide a clever way of keeping a history of files, should you need to recover an older copy or even a deleted file. For this reason, many administrators take snapshots often (e.g. every 15 minutes), store them for a period of time (e.g. for a month), and store them on another system (e.g. using Replication Tasks). Such a strategy allows the administrator to roll the system back to a specific time or, if there is a catastrophic loss, an off-site snapshot can restore the system up to the last snapshot interval.

Before you can create a snapshot, you need to have an existing ZFS volume. How to create a volume is described in ZFS Volume Manager.

8.1.1. Creating a Periodic Snapshot Task

To create a periodic snapshot task, click Storage –> Periodic Snapshot Tasks –> Add Periodic Snapshot which will open the screen shown in Figure 8.1a. Table 8.1a summarizes the fields in this screen.


if you just need a one-time snapshot, instead use Storage –> Volumes –> View Volumes and click the “Create Snapshot” button for the volume or dataset that you wish to snapshot.

Figure 8.1a: Creating a ZFS Periodic Snapshot


Table 8.1a: Options When Creating a Periodic Snapshot

Setting Value Description
Enabled checkbox uncheck to disable the scheduled replication task without deleting it
Volume/Dataset drop-down menu select an existing ZFS volume, dataset, or zvol; if you select a volume, separate snapshots will also be created for each of its datasets
Recursive checkbox select this box to take separate snapshots of the volume/dataset and each of its child datasets; if unchecked, only one snapshot is taken of the volume/dataset specified in “Volume/Dataset”
Lifetime integer and drop-down menu how long to keep the snapshot on this system; if the snapshot is replicated, it is not removed from the receiving system when the lifetime expires
Begin drop-down menu do not create snapshots before this time of day
End drop-down menu do not create snapshots after this time of day
Interval drop-down menu how often to take snapshot between the “Begin” and “End” times
Weekday checkboxes which days of the week to take snapshots

If the “Recursive” box is checked, you do not need to create snapshots for every dataset individually as they are included in the snapshot. The downside is that there is no way to exclude certain datasets from being included in a recursive snapshot.

Once you click the “OK” button, a snapshot will be taken and this task will be repeated according to your settings.

8.1.2. Managing Periodic Snapshot Tasks

After creating a periodic snapshot task, an entry for the snapshot task will be added to “View Periodic Snapshot Tasks”, as seen in the example in Figure 8.1b. Click an entry to access its “Modify” and “Delete” buttons.

If you click the “ZFS Snapshots” tab (above the “Add Periodic Snapshot” button), you can review the listing of available snapshots. An example is shown in Figure 8.1c.


if snapshots do not appear, check that the current time does not conflict with the begin, end, and interval settings. If the snapshot was attempted but failed, an entry will be added to /var/log/messages. This log file can be viewed in `Shell`_.

Figure 8.1b: View Periodic Snapshot Tasks


Figure 8.1c: Viewing Available Snapshots


The most recent snapshot for a volume or dataset will be listed last and will have 3 icons. The icons associated with a snapshot allow you to:

Clone Snapshot: will prompt for the name of the clone to create. The clone will be a writable copy of the snapshot. Since a clone is really a dataset which can be mounted, the clone will appear in the “Active Volumes” tab, instead of the “Periodic Snapshots” tab, and will have the word clone in its name.

Destroy Snapshot: a pop-up message will ask you to confirm this action. Child clones must be destroyed before their parent snapshot can be destroyed. While creating a snapshot is instantaneous, deleting a snapshot can be I/O intensive and can take a long time, especially when deduplication is enabled. In order to delete a block in a snapshot, ZFS has to walk all the allocated blocks to see if that block is used anywhere else; if it is not, it can be freed.

Rollback Snapshot: a pop-up message will ask if you are sure that you want to rollback to this snapshot state. If you click “Yes”, any files that have changed since the snapshot was taken will be reverted back to their state at the time of the snapshot.


rollback is a potentially dangerous operation and will cause any configured replication tasks to fail as the replication system uses the existing snapshot when doing an incremental backup. If you do need to restore the data within a snapshot, the recommended steps are:

  1. Clone the desired snapshot.
  2. Share the clone with the share type or service running on the TrueNAS® system.
  3. Once users have recovered the needed data, destroy the clone in the Active Volumes tab.

This approach will never destroy any on-disk data and has no impact on replication.

Periodic snapshots can be configured to appear as `Shadow Copies`_ in newer versions of Windows Explorer. Users can access the files in the shadow copy using Explorer without requiring any interaction with the TrueNAS® graphical administrative interface.

The “ZFS Snapshots” screen allows you to create filters to view snapshots by selected criteria. To create a filter, click the “Define filter” icon (near the text “No filter applied”). When creating a filter:

  • select the column or leave the default of Any Column.
  • select the condition. Possible conditions are: contains (default), is, starts with, ends with, does not contain, is not, does not start with, does not end with, and is empty.
  • input a value that meets your view criteria.
  • click the “Filter” button to save your filter and exit the define filter screen. Alternately, click the “+” button to add another filter.

If you create multiple filters, select the filter you wish to use before leaving the define filter screen. Once a filter is selected, the “No filter applied” text will change to “Clear filter”. If you click “Clear filter”, a pop-up message will indicate that this will remove the filter and all available snapshots will be listed.

8.2. Replication Tasks

A replication task allows you to automate the copy of ZFS snapshots to another system over an encrypted connection. This allows you to create an off-site backup of a ZFS dataset or pool.

This section will refer to the system generating the ZFS snapshots as PUSH and the system to receive a copy of the ZFS snapshots as PULL.

Before you can configure a replication task, the following pre-requisites must be met:

  • a ZFS volume must exist on both PUSH and PULL.
  • a periodic snapshot task must be created on PUSH. You will not be able to create a replication task before the first snapshot exists.
  • the SSH service must be enabled on PULL. The first time the service is enabled, it will generate the required SSH keys.

A replication task uses the following keys:

  • /data/ssh/replication.pub: the RSA public key used for authenticating the PUSH replication user. This key needs to be copied to the replication user account on PULL.
  • /etc/ssh/ssh_host_rsa_key.pub: the RSA host public key of PULL used to authenticate the receiving side in order to prevent a man-in-the-middle attack. This key needs to be copied to the replication task on PUSH.

This section will demonstrate how to configure a replication task between the following two TrueNAS® systems:

  • will be referred to as PUSH. This system has a periodic snapshot task for the ZFS dataset /mnt/local/data.
  • will be referred to as PULL . This system has an existing ZFS volume named /mnt/remote which will store the pushed snapshots.

8.2.1. Configure PULL

A copy of the public key for the replication user on PUSH needs to be pasted to the public key of the replication user on the PULL system.

To obtain a copy of the replication key: on PUSH go to Storage –> View Replication Tasks. Click the “View Public Key” button and copy its contents. An example is shown in Figure 8.2a.

Figure 8.2a: Copy the Replication Key


Go to PULL and click Account –> Users –> View Users. Click the “Modify User” button for the user account you will be using for replication (by default this is the root user). Paste the copied key into the “SSH Public Key” field and click “OK”. If a key already exists, append the new text after the existing key.

On PULL, ensure that the SSH service is enabled in Services –> Control Services. Start it if it is not already running.

8.2.2. Configure PUSH

On PUSH, verify that a periodic snapshot task has been created and that at least one snapshot is listed in Storage –> Periodic Snapshot Tasks –> View Periodic Snapshot Tasks –> ZFS Snapshots.

To create the replication task, click Storage –> Replication Tasks –> Add Replication Task. In the screen shown in Figure 8.2b, input the required configuration. For this example:

  • the “Volume/Dataset” is local/data
  • the “Remote ZFS Volume/Dataset” is remote
  • the “Remote hostname” is
  • the “Begin” and “End” times are at their default values, meaning that replication will occur whenever a snapshot is created
  • once the “Remote hostname” is input, click the “SSH Key Scan” button; assuming the address is reachable and the SSH service is running on PULL, its key will automatically be populated to the Remote hostkey box

Figure 8.2b: Adding a Replication Task


Table 8.2a summarizes the available options in the Add Replication Task screen.

Table 8.2a: Adding a Replication Task

Setting Value Description
Enabled checkbox uncheck to disable the scheduled replication task without deleting it
Volume/Dataset drop-down menu the ZFS volume or dataset on PUSH containing the snapshots to be replicated; the drop-down menu will be empty if a snapshot does not already exist
Remote ZFS Volume/Dataset string the ZFS volume on PULL that will store the snapshots; /mnt/ is assumed and should not be included in the path
Recursively replicate checkbox if checked will replicate child datasets and replace previous snapshot stored on PULL
Initialize remote side checkbox does a reset once operation which destroys the replication data on PULL before reverting to normal operation; use this option if replication gets stuck
Limit (kB/s) integer limits replication speed to specified value in kilobytes/second; default of 0 is unlimited
Begin drop-down menu the replication can not start before this time; the times selected in the “Begin” and “End” fields set the replication window for when replication can occur
End drop-down menu the replication must start by this time; once started, replication will occur until it is finished (see NOTE below)
Remote hostname string IP address or DNS name of PULL
Remote port string must match port being used by SSH service on PULL
Dedicated User Enabled checkbox allows a user account other than root to be used for replication
Dedicated User drop-down menu only available if “Dedicated User Enabled” is checked; select the user account to be used for replication
Enable High Speed Ciphers checkbox note that the cipher is quicker because it has a lower strength
Remote hostkey string use the SSH Key Scan button to retrieve the public key of PULL

By default, replication occurs when snapshots occur. For example, if snapshots are scheduled for every 2 hours, replication occurs every 2 hours. The “Begin” and “End” times can be used to create a window of time where replication occurs. Change the default times (which allow replication to occur at any time of the day a snapshot occurs) if snapshot tasks are scheduled during office hours but the replication itself should occur after office hours. For the “End” time, consider how long replication will take so that it finishes before the next day’s office hours begin.

Once the replication task is created, it will appear in the “View Replication Tasks” of PUSH.

PUSH will immediately attempt to replicate its latest snapshot to PULL. If the replication is successful, the snapshot will appear in the Storage –> Periodic Snapshot Tasks –> View Periodic Snapshot Tasks –> ZFS Snapshots tab of PULL. If the snapshot is not replicated, see the next section for troubleshooting tips.

8.2.3. Troubleshooting Replication

If you have followed all of the steps above and have PUSH snapshots that are not replicating to PULL, check to see if SSH is working properly. On PUSH, open `Shell`_ and try to ssh into PULL. Replace hostname_or_ip with the value for PULL:

ssh -vv -i /data/ssh/replication hostname_or_ip

This command should not ask for a password. If it asks for a password, SSH authentication is not working. Go to Storage –> Replication Tasks –> View Replication Tasks and click the “View Public Key” button. Make sure that it matches one of the values in /~/.ssh/authorized_keys on PULL, where ~ represents the home directory of the replication user.

Also check /var/log/auth.log on PULL and /var/log/messages on PUSH to see if either log gives an indication of the error.

If the key is correct and replication is still not working, try deleting all snapshots on PULL except for the most recent one. In Storage –> Periodic Snapshot Tasks –> View Periodic Snapshot Tasks –> ZFS Snapshots check the box next to every snapshot except for the last one (the one with 3 icons instead of 2), then click the global “Destroy” button at the bottom of the screen.

Once you have only one snapshot, open Shell on PUSH and use the zfs send command. To continue our example, the ZFS snapshot on the local/data dataset of PUSH is named auto-20110922.1753-2h, the IP address of PULL is, and the ZFS volume on PULL is remote. Note that the @ is used to separate the volume/dataset name from the snapshot name.

zfs send local/data@auto-20110922.1753-2h | ssh -i /data/ssh/replication zfs receive local/data@auto-20110922.1753-2h


if this command fails with the error “cannot receive new filesystem stream: destination has snapshots”, check the box “initialize remote side for once” in the replication task and try again. If the zfs send command still fails, you will need to open Shell on PULL and use the zfs destroy -R volume_name@snapshot_name command to delete the stuck snapshot. You can then use the zfs list -t snapshot on PULL to confirm if the snapshot successfully replicated.

After successfully transmitting the snapshot, recheck again after the time period between snapshots lapses to see if the next snapshot successfully transmitted. If it is still not working, you can manually send an incremental backup of the last snapshot that is on both systems to the current one with this command:

zfs send local/data@auto-20110922.1753-2h | ssh -i /data/ssh/replication zfs receive local/data@auto-20110922.1753-2h

8.3. Volumes

Since the storage disks are separate from the TrueNAS® operating system, you do not actually have a NAS (network-attached storage) system until you configure your disks into at least one volume.


in ZFS terminology, the storage that is managed by ZFS is referred to as a pool. When configuring the ZFS pool using the TrueNAS® graphical interface, the term volume is used to refer to a ZFS pool.

Proper storage design is important for any NAS. It is recommended that you read through this entire chapter first, before configuring your storage disks, so that you are aware of all of the possible features, know which ones will benefit your setup most, and are aware of any caveats or hardware restrictions.

8.3.1. Auto Importing Volumes

If you click Storage –> Volumes –> Auto Import Volume, you can configure TrueNAS® to use an existing software UFS or ZFS RAID volume. This action is typically performed when an existing TrueNAS® system is re-installed (rather than upgraded). Since the operating system is separate from the disks, a new installation does not affect the data on the disks; however, the new operating system needs to be configured to use the existing volume.

Supported volumes are UFS GEOM stripes (RAID0), UFS GEOM mirrors (RAID1), UFS GEOM RAID3, as well as existing ZFS pools. UFS RAID5 is not supported as it is an unmaintained summer of code project which was never integrated into FreeBSD.

The import of existing GELI-encrypted ZFS pools is also supported. However, the pool must be decrypted before it can be imported.

Figure 8.3a shows the initial pop-up window that appears when you select to auto import a volume. If you are importing a UFS RAID or an existing, unencrypted ZFS pool, select “No: Skip to import”.

Figure 8.3a: Initial Auto Import Volume Screen


A drop-down menu will then show which software RAID volumes are available for import. Select the volume and click the “OK” button to import the volume.

TrueNAS® will not import a dirty volume. If an existing UFS RAID does not show in the drop-down menu, you will need to fsck the volume.

If an existing ZFS pool does not show in the drop-down menu, run :command:zpool import` from `Shell`_ to import the pool.

If you plan to physically install ZFS formatted disks from another system, be sure to export the drives on that system to prevent an “in use by another machine” error during the import.

If you suspect that your hardware is not being detected, run camcontrol devlist from Shell. If the disk does not appear in the output, check to see if the controller driver is supported or if it needs to be loaded by creating a `tunable`_. Auto Importing a GELI-Encrypted ZFS Pool

If you are importing an existing GELI-encrypted ZFS pool, you must decrypt the disks before importing the pool. In Figure 8.3a, select “Yes: Decrypt disks” to access the screen shown in Figure 8.3b.

Figure 8.3b Decrypting the Disks Before Importing the ZFS Pool


Select the disks in the encrypted pool, browse to the location of the saved encryption key, input the passphrase associated with the key, then click “OK” to decrypt the disks.


the encryption key is required to decrypt the pool. If the pool can not be decrypted, it can not be re-imported after a failed upgrade or lost configuration. This means that it is very important to save a copy of the key and to remember the passphrase that was configured for the key. The `View Volumes`_ screen is used to manage the keys for encrypted volumes.

Once the pool is decrypted, it should appear in the “import” drop-down menu. Click the “OK” button to finish the volume import.

8.3.2. Importing Volumes

The Volume –> Import Volume screen, shown in Figure 8.3c, is used to import a single disk or partition that has been formatted with a supported filesystem. TrueNAS® supports the import of disks that have been formatted with UFS, NTFS, MSDOS, or EXT2. The import is meant to be a temporary measure in order to copy the data from a disk to a volume. Only one disk can be imported at a time.

Figure 8.3c: Importing a Volume


Input a name for the volume, use the drop-down menu to select the disk or partition that you wish to import, and select the type of filesystem on the disk.

Before importing a disk, be aware of the following caveats:

  • TrueNAS® will not import a dirty filesystem. If a supported filesystem does not show in the drop-down menu, you will need to fsck or run a disk check on the filesystem.
  • TrueNAS® can not import dynamic NTFS volumes.
  • if an NTFS volume will not import, try ejecting the volume safely from a Windows system. This will fix some journal files that are required to mount the drive.

8.3.3. ZFS Volume Manager

If you have unformatted disks or wish to overwrite the filesystem (and data) on your disks, use the “ZFS Volume Manager” to format the desired disks into a ZFS pool.

If you click on Storage –> Volumes –> ZFS Volume Manager, you will see a screen similar to the example shown in Figure 8.3d.

Figure 8.3d: Creating a ZFS Pool Using Volume Manager


Table 8.3a summarizes the configuration options of this screen.

Table 8.3a: Options When Creating a ZFS Volume

To configure the pool, drag the slider to select the desired number of disks. The ZFS Volume Manager will automatically select the optimal configuration and the resulting storage capacity, which takes swap into account, will be displayed. If you wish to change the layout or the number of disks, use the mouse to drag the slider to the desired volume layout. The drop-down menu showing the optimal configuration can also be clicked to change the configuration, though the GUI will turn red if the selected configuration is not recommended.


for performance and capacity reasons, this screen will not allow you to create a volume from disks of differing sizes. While it is not recommended, it is possible to create a volume in this situation by using the “Manual setup” button and following the instructions in Manual Volume Creation.

ZFS Volume Manager will allow you to save save a non-optimal configuration. It will still work, but will perform less efficiently than an optimal configuration. However, the GUI will not allow you to select a configuration if the number of disks selected is not enough to create that configuration. Click the tool tip icon to access a link to this documentation.

The Add Volume button warns that creating a volume will destroys any existing data on the selected disk(s). In other words, creating a new volume reformats the selected disks. If your intent is to not overwrite the data on an existing volume, see if the volume format is supported by the auto-import_ or import actions. If so, perform the supported action instead. If the current storage format is not supported, you will need to backup the data to an external media, format the disks, then restore the data to the new volume.

The ZFS Volume Manager will automatically select the optimal layout for the new pool, depending upon the number of disks selected. The following formats are supported:

  • Stripe: requires at least one disk
  • Mirror: requires at least two disks
  • RAIDZ1: requires at least three disks
  • RAIDZ2: requires at least four disks
  • RAIDZ3: requires at least five disks
  • log device: add a dedicated log device (slog)
  • cache device: add a dedicated cache device

If you have more than five disks and are using ZFS, consider the number of disks to use for best performance and scalability.

Depending upon the size and number of disks, the type of controller, and whether or not encryption is selected, creating the volume may take some time. Once the volume is created, the screen will refresh and the new volume will be listed under Storage –> Volumes. Encryption

TrueNAS® supports GELI full disk encryption when creating ZFS volumes. It is important to understand the following when considering whether or not encryption is right for your TrueNAS® system:

  • This is not the encryption method used by Oracle ZFSv30. That version of ZFS has not been open sourced and is the property of Oracle.
  • This is full disk encryption and not per-filesystem encryption. The underlying drives are first encrypted, then the pool is created on top of the encrypted devices.
  • This type of encryption is primarily targeted at users who store sensitive data and want to retain the ability to remove disks from the pool without having to first wipe the disk’s contents.
  • This design is only suitable for safe disposal of disks independent of the encryption key. As long as the key and the disks are intact, the system is vulnerable to being decrypted. The key should be protected by a strong passphrase and any backups of the key should be securely stored.
  • On the other hand, if the key is lost, the data on the disks is inaccessible. Always backup the key!
  • The encryption key is per ZFS volume (pool). If you create multiple pools, each pool has its own encryption key.
  • As data is written, it is automatically encrypted and as data is read, it is decrypted on the fly. There should be very little, if any, degradation in performance when using encryption.
  • Data in the ARC cache and the contents of RAM are unencrypted.
  • Swap is always encrypted, even on unencrypted volumes.
  • There is no way to convert an existing, unencrypted volume. Instead, the data must be backed up, the existing pool must be destroyed, a new encrypted volume must be created, and the backup restored to the new volume.
  • Hybrid pools are not supported. In other words, newly created vdevs must match the existing encryption scheme. When Extending a ZFS Volume, Volume Manager will automatically encrypt the new vdev being added to the existing encrypted pool.


the encryption facility used by TrueNAS® is designed to protect against physical theft of the disks. It is not designed to protect against unauthorized software access. Ensure that only authorized users have access to the administrative GUI and that proper permissions are set on shares if sensitive data stored on the system. Creating an Encrypted Volume

To create an encrypted volume, check the “Encryption” box shown in Figure 8.3d. Input the volume name, select the disks to add to the volume, and click the “Add Volume” button to make the encrypted volume.

Once the volume is created, it is extremely important to set a passphrase on the key, make a backup of the key, and create a recovery key. Without these, it is impossible to re-import the disks at a later time.

To perform these tasks, go to Storage –> Volumes -> View Volumes. This screen is shown in Figure 8.3n.

To set a passphrase on the key, click the volume name and then the “Create Passphrase” button (the key shaped icon in Figure 8.3n). You will be prompted to input the password used to access the TrueNAS® administrative GUI, and then to input and repeat the desired passphrase. Unlike a password, a passphrase can contain spaces and is typically a series of words. A good passphrase is easy to remember (like the line to a song or piece of literature) but hard to guess (people who know you should not be able to guess the passphrase).

When you set the passphrase, a warning message will remind you to create a new recovery key as a new passphrase needs a new recovery key. This way, if the passphrase is forgotten, the associated recovery key can be used instead. To create the recovery key, click the “Add recovery key” button (second last key icon in Figure 8.3n). This screen will prompt you to input the password used to access the TrueNAS® administrative GUI and then to select the directory in which to save the key. Note that the recovery key is saved to the client system, not on the TrueNAS® system.

Finally, download a copy of the encryption key, using the “Download key” button (the key icon with a down arrow in Figure 8.3n). Again, the encryption key is saved to the client system, not on the TrueNAS® system. You will be prompted to input the password used to access the TrueNAS® administrative GUI before the selecting the directory in which to store the key.

The passphrase, recovery key, and encryption key need to be protected. Do not reveal the passphrase to others. On the system containing the downloaded keys, take care that that system and its backups are protected. Anyone who has the keys has the ability to re-import the disks should they be discarded or stolen. Manual Volume Creation

The “Manual Setup” button shown in Figure 8.3d can be used to create a non-optimal ZFS volume. While this is not recommended, it can, for example, be used to create a volume containing disks of different sizes or to put more than the recommended number of disks into a vdev.


when using disks of differing sizes, the volume is limited by the size of the smallest disk. When using more disks than are recommended for a vdev, you increase resilvering time and the risk that more than the allowable number of disks will fail before a resilver completes. For these reasons, it is recommended to instead let the ZFS Volume Manager create an optimal pool for you, as described in ZFS Volume Manager, using same-size disks.

Figure 8.3e shows the “Manual Setup” screen and Table 8.3b summarizes the available options.

Figure 8.3e: Creating a Non-Optimal ZFS Volume


Table 8.3b: Manual Setup Options

Setting Value Description
Volume name string ZFS volumes must conform to these naming conventions ; it is recommended to choose a name that will stick out in the logs (e.g. not data or freenas)
Encryption checkbox read the section on Encryption before choosing to use encryption
Member disks list highlight desired number of disks from list of available disks
Deduplication drop-down menu choices are Off, Verify, and On; carefully consider the section on Deduplication before changing this setting
ZFS Extra bullet selection used to specify if disk is used for storage (None), a log device, a cache device, or a spare

8.3.4. Extending a ZFS Volume

The “Volume to extend” drop-down menu in Storage –> Volumes –> ZFS Volume Manager, shown in Figure 8.3f, can be used to add additional disks to an existing ZFS volume. This drop-down empty will be empty if an existing ZFS volume does not exist.

Figure 8.3f: Extending a Volume



if the existing volume is encrypted, a warning message will remind you that the operation of extending a volume will reset the passphrase and recovery key. After extending the volume, you should immediately recreate both as described in Creating an Encrypted Volume.

Once an existing volume has been selected from the drop-down menu, drag and drop the desired disk(s) and select the desired volume layout. For example you can:

  • select an SSD or disk with a volume layout of Log (ZIL) to add a log device to the ZFS pool. Selecting 2 SSDs or disks will mirror the log device.
  • select an SSD or disk with a volume layout of Cache (L2ARC) to add a cache device to the ZFS pool.
  • add additional disks to increase the capacity of the ZFS pool. The caveats to doing this are described below.

When adding disks to increase the capacity of a volume, ZFS supports the addition of virtual devices, known as vdevs, to an existing ZFS pool. A vdev can be a single disk, a stripe, a mirror, a RAIDZ1, RAIDZ2, or a RAIDZ3. Once a vdev is created, you can not add more drives to that vdev; however, you can stripe a new vdev (and its disks) with the same type of existing vdev in order to increase the overall size of ZFS the pool. In other words, when you extend a ZFS volume, you are really striping similar vdevs. Here are some examples:

  • to extend a ZFS stripe, add one or more disks. Since there is no redundancy, you do not have to add the same amount of disks as the existing stripe.
  • to extend a ZFS mirror, add the same number of drives. The resulting striped mirror is a RAID 10. For example, if you have 10 drives, you could start by creating a mirror of two drives, extending this mirror by creating another mirror of two drives, and repeating three more times until all 10 drives have been added.
  • to extend a three drive RAIDZ1, add three additional drives. The result is a RAIDZ+0, similar to RAID 50 on a hardware controller.
  • to extend a RAIDZ2 requires a minimum of four additional drives. The result is a RAIDZ2+0, similar to RAID 60 on a hardware controller.

If you try to add an incorrect number of disks to the existing vdev, an error message will appear, indicating the number of disks that are needed. You will need to select the correct number of disks in order to continue.

8.3.5. Creating ZFS Datasets

An existing ZFS volume can be divided into datasets. Permissions, compression, deduplication, and quotas can be set on a per dataset basis, allowing more granular control over access to storage data. A dataset is similar to a folder in that you can set permissions; it is also similar to a filesystem in that you can set properties such as quotas and compression as well as create snapshots.


ZFS provides thick provisioning using quotas and thin provisioning using reserved space.

If you select an existing ZFS volume –> Create ZFS Dataset, you will see the screen shown in Figure 8.3g.

Once a dataset is created, you can click on that dataset and select Create ZFS Dataset, thus creating a nested dataset, or a dataset within a dataset. You can also create a zvol within a dataset. When creating datasets, double-check that you are using the Create ZFS Dataset option for the intended volume or dataset. If you get confused when creating a dataset on a volume, click all existing datasets to close them–the remaining Create ZFS Dataset will be for the volume.

Table 8.3c summarizes the options available when creating a ZFS dataset. Some settings are only available in Advanced Mode. To see these settings, either click the “Advanced Mode” button or configure the system to always display these settings by checking the box “Show advanced fields by default” in System –> Settings –> Advanced. These attributes can also be changed after dataset creation in Storage –> Volumes –> View Volumes`.

Figure 8.3g: Creating a ZFS Dataset


Table 8.3c: ZFS Dataset Options

Setting Value Description
Dataset Name string mandatory
Compression Level drop-down menu see Compression for a comparison of the available algorithms
Share Type drop-down menu select the type of share that will be used on the dataset; choices are UNIX for an NFS share, Windows for a CIFS share, or Mac for an AFP share
Enable atime Inherit, On, or Off controls whether the access time for files is updated when they are read; setting this property to Off avoids producing log traffic when reading files and can result in significant performance gains
Quota for this dataset integer only available in “Advanced Mode”; default of 0 is off; can specify M (megabyte), G (gigabyte), or T (terabyte) as in 20G for 20 GB, can also include a decimal point (e.g. 2.8G)
Quota for this dataset and all children integer only available in “Advanced Mode”; default of 0 is off; can specify M (megabyte), G (gigabyte), or T (terabyte) as in 20G for 20 GB
Reserved space for this dataset integer only available in “Advanced Mode”; default of 0 is unlimited (besides hardware); can specify M (megabyte), G (gigabyte), or T (terabyte) as in 20G for 20 GB
Reserved space for this dataset and all children integer only available in “Advanced Mode”; default of 0 is unlimited (besides hardware); can specify M (megabyte), G (gigabyte), or T (terabyte) as in 20G for 20 GB
ZFS Deduplication drop-down menu read the section on Deduplication before making a change to this setting
Record Size drop-down menu only available in “Advanced Mode”; while ZFS automatically adapts the record size dynamically to adapt to data, if the data has a fixed size (e.g. a database), setting the “Record Size” may result in better performance Deduplication

The “ZFS Deduplication” option warns that enabling dedup may have drastic performance implications and that compression should be used instead. This article provides a good description of the value v.s. cost considerations for deduplication.

Unless you have a lot of RAM and a lot of duplicate data, do not change the default deduplication setting of “Off”. The dedup tables used during deduplication need ~8 GB of RAM per 1TB of data to be deduplicated. For performance reasons, consider using compression rather than turning this option on.

If deduplication is changed to On, duplicate data blocks are removed synchronously. The result is that only unique data is stored and common components are shared among files. If deduplication is changed to Verify, ZFS will do a byte-to-byte comparison when two blocks have the same signature to make sure that the block contents are identical. Since hash collisions are extremely rare, verify is usually not worth the performance hit.


once deduplication is enabled, the only way to disable it is to use the zfs set dedup=off dataset_name command from `Shell`_. However, any data that is already stored as deduplicated will not be un-deduplicated as only newly stored data after the property change will not be deduplicated. The only way to remove existing deduplicated data is to copy all of the data off of the dataset, set the property to off, then copy the data back in again. Alternately, create a new dataset with the ZFS Deduplication left as disabled, copy the data to the new dataset, and destroy the original dataset. Compression

Most media (e.g. .mp3, .mp4, .avi) is already compressed, meaning that you will increase CPU utilization for no gain if you store these files on a compressed dataset. However, if you have raw .wav rips of CDs or .vob rips of DVDs, you will see a performance gain using a compressed dataset. When selecting a compression type, you need to balance performance with the amount of compression. The following compression algorithms are supported:

  • lz4: recommended compression method as it allows compressed datasets to operate at near real-time speed.
  • gzip: varies from levels 1 to 9 where gzip fastest (level 1) gives the least compression and gzip maximum (level 9) provides the best compression but is discouraged due to its performance impact.
  • zle: fast and simple algorithm to eliminate runs of zeroes.
  • lzjb: provides decent data compression, but is considered deprecated as lz4 provides much better performance.

8.3.6. Creating a zvol

A zvol is a feature of ZFS that creates a block device over ZFS. This allows you to use a zvol as an iSCSI device extent as described in `Extents>`_.

To create a zvol, select an existing ZFS volume or dataset –> Create zvol which will open the screen shown in Figure 8.3h.

The configuration options are described in Table 8.3d. Some settings are only available in Advanced Mode. To see these settings, either click the “Advanced Mode” button or configure the system to always display these settings by checking the box “Show advanced fields by default” in System –> Settings –> Advanced.

Figure 8.3h Creating a zvol


Table 8.3d: zvol Configuration Options

Setting Value Description
zvol Name string input a name for the zvol
Size for this zvol integer specify size and value such as 10G
Compression level drop-down menu default of Inherit means it will use the same compression level as the existing zpool used to create the zvol
Sparse volume checkbox used to provide thin provisioning; if this option is selected, writes will fail when the pool is low on space
Block size integer only available in “Advanced Mode”; valid size is any power of 2 from 512b to 128kb with a default size of 8kb; can be set to match the block size of the filesystem which will be formatted onto the iSCSI target

8.3.7. Viewing Disks

Storage –> Volumes –> View Disks allows you to view all of the disks recognized by the TrueNAS® system. An example is shown in Figure 8.3i.

Figure 8.3i: Viewing Disks


For each device, the current configuration of the options described in Table 8.3e is displayed. Click a disk’s entry and then its “Edit” button to change its configuration.

Clicking a disk’s entry will also display its “Wipe” button which can be used to blank a disk while providing a progress bar of the wipe’s status. Use this option before discarding a disk.


should a disk’s serial number not be displayed in this screen, use the smartctl command within `Shell`_. For example, to determine the serial number of disk ada0, type smartctl -a /dev/ada0 | grep Serial.

8.3.8. Viewing Volumes

If you click Storage –> Volumes –> View Volumes, you can view and further configure existing volumes, ZFS datasets, and zvols. The example shown in Figure 8.3j demonstrates one ZFS volume with two datasets and one zvol.

Buttons are provided to provide quick access to ZFS Volume Manager, and for Importing Volumes, `Importing Volumes>`_, Viewing Disks, and Viewing Enclosures.

Figure 8.3j: Viewing Volumes


If you click the entry for a ZFS volume, eight icons will appear at the bottom of the screen. In order from left to right, these icons allow you to:

  1. Detach Volume: allows you to either detach a disk before removing it from the system (also known as a ZFS export) or to delete the contents of the volume, depending upon the choice you make in the screen that pops up when you click this button. The pop-up message, seen in Figure 8.3k, will show the current used space, provide the check box “Mark the disks as new (destroy data)”, prompt you to make sure that you want to do this, warn you if the volume has any associated shares and ask if you wish to delete them, and the browser will turn red to alert you that you are about to do something that will make the data inaccessible. If you do not check the box to mark the disks as new, the volume will be exported (ZFS volumes only). This means that the data is not destroyed and the volume can be re-imported at a later time. If you will be moving a ZFS drive from one system to another, perform this export action first. This operation flushes any unwritten data to disk, writes data to the disk indicating that the export was done, and removes all knowledge of the pool from the system. If you do check the box to mark the disks as new, the volume and all of its data, datasets, and zvols will be destroyed and the underlying disks will be returned to their raw state.
  2. Scrub Volume: ZFS scrubs and how to schedule them are described in more detail in ZFS Scrubs. This button allows you to manually initiate a scrub. A scrub is I/O intensive and can negatively impact performance, meaning that you should not initiate one while the system is busy. A cancel button is provided should you need to cancel a scrub.


if you do cancel a scrub, the next scrub will start over from the beginning, not where the cancelled scrub left off.

Figure 8.3k: Detaching or Deleting a Volume


  1. Edit ZFS Options: allows you to edit the volume’s compression level, atime setting, dataset quota, and reserved space for quota. If compression is newly enabled on a volume or dataset that already contains data, existing files will not be compressed until they are modified as compression is only applied when a file is written.
  2. Create ZFS Dataset: allows you to create a dataset.
  3. Create zvol: allows you to create a zvol to use as an iSCSI device extent.
  4. Change Permissions: allows you to edit the volume’s user, group, Unix rwx permissions, type of ACL, and to enable recursive permissions on the volume’s subdirectories.
  5. Create Snapshot: allows you to configure the snapshot’s name and whether or not it is recursive before manually creating a one-time snapshot. If you wish to schedule the regular creation of snapshots, instead create Periodic Snapshot Tasks.
  6. Volume Status: as seen in the example in Figure 8.3l, this screen shows the device name and status of each disk in the ZFS pool as well as any read, write, or checksum errors. It also indicates the status of the latest ZFS Scrubs. If you click the entry for a device, buttons will appear to edit the device’s options (shown in Figure 8.3m ), offline the device, or replace the device (as described in Replacing a Failed Drive).

Figure 8.3l: Volume Status


If you click a disk in “Volume Status” and click its “Edit Disk” button, you will see the screen shown in Figure 8.3m. Table 8.3e summarizes the configurable options.

Figure 8.3m: Editing a Disk


Table 8.3e: Disk Options

Setting Value Description
Name string read-only value showing FreeBSD device name for disk
Serial string read-only value showing the disk’s serial number
Description string optional
HDD Standby drop-down menu indicates the time of inactivity (in minutes) before the drive enters standby mode in order to conserve energy; this forum post demonstrates how to determine if a drive has spun down
Advanced Power Management drop-down menu default is Disabled, can select a power management profile from the menu
Acoustic Level drop-down menu default is Disabled, can be modified for disks that understand AAM
Enable S.M.A.R.T checkbox enabled by default if the disk supports S.M.A.R.T.; unchecking this box will disable any configured S.M.A.R.T. Tests for the disk
S.M.A.R.T. extra options string smartctl(8) options

A ZFS dataset only has five icons as the “scrub volume”, “create ZFS volume”, and “volume status” buttons only apply to volumes. In a dataset, the “Detach Volume” button is replaced with the “Destroy Dataset” button. If you click the “Destroy Dataset” button, the browser will turn red to indicate that this is a destructive action. The pop-up warning message will warn that destroying the dataset will delete all of the files and snapshots of that dataset. Key Management for Encrypted Volumes

If you check the “Enable full disk encryption” box during the creation of a ZFS volume, five encryption icons will be added to the icons that are typically seen when viewing a volume. An example is seen in Figure 8.3n.

Figure 8.3n: Encryption Icons Associated with an Encrypted ZFS Volume


These icons are used to:

Create/Change Passphrase: click this icon to set and confirm the passphrase associated with the GELI encryption key. Remember this passphrase as you can not re-import an encrypted volume without it. In other words, if you forget the passphrase, it is possible for the data on the volume to become inaccessible. An example would be a failed USB stick that requires a new installation on a new USB stick and a re-import of the existing pool, or the physical removal of disks when moving from an older hardware system to a new system. Protect this passphrase as anyone who knows it could re-import your encrypted volume, thus thwarting the reason for encrypting the disks in the first place.

When you click this icon, a red warning is displayed: Remember to add a new recovery key as this action invalidates the previous recovery key. Setting a passphrase invalidates the existing key. Once you set the passphrase, immediately click the “Add recovery key” button to create a new recovery key. Once the passphrase is set, the name of this icon will change to “Change Passphrase”.

Download Key: click this icon to download a backup copy of the GELI encryption key. Since the GELI encryption key is separate from the TrueNAS® configuration database, it is highly recommended to make a backup of the key. If the key is every lost or destroyed and there is no backup key, the data on the disks is inaccessible.

Encryption Re-key: generates a new GELI encryption key. Typically this is only performed when the administrator suspects that the current key may be compromised. This action also removes the current passphrase.

Add recovery key: generates a new recovery key and prompts for a location to download a backup copy of the recovery key. This recovery key can be used if the passphrase is forgotten. Always immediately add a recovery key whenever the passphrase is changed.

Remove recover key: Typically this is only performed when the administrator suspects that the current recovery key may be compromised. Immediately create a new passphrase and recovery key.

Each of these icons will prompt for the password used to access the TrueNAS® administrative GUI.

8.3.9. Setting Permissions

Setting permissions is an important aspect of configuring volumes. The graphical administrative interface is meant to set the initial permissions for a volume or dataset in order to make it available as a share. Once a share is available, the client operating system should be used to fine-tune the permissions of the files and directories that are created by the client.

`Sharing`_ contains configuration examples for several types of permission scenarios. This section provides an overview of the screen that is used to set permissions.

Once a volume or dataset is created, it will be listed by its mount point name in Storage –> Volumes –> View Volumes. If you click the “Change Permissions” icon for a specific volume/dataset, you will see the screen shown in Figure 8.3o. Table 8.3f summarizes the options in this screen.

Figure 8.3o: Changing Permissions on a Volume or Dataset


Table 8.3f: Options When Changing Permissions

When in doubt, or if you have a mix of operating systems in your network, select Unix ACLs as all clients understand them. Windows/Mac ACLs are appropriate when the network contains only Windows and/or Mac clients and are the preferred option within an Active Directory domain. Windows/Mac ACLs add a superset of permissions that augment those provided by Unix ACLs. While Windows and Mac clients also understand Unix ACLs, they won’t benefit from the extra permissions provided by Active Directory and Windows/Mac ACLs when Unix ACLs are used.

If you change your mind about the type of ACL, you do not have to recreate the volume. That is, existing data is not lost if the type of ACL is changed. However, if you change from Windows/Mac ACLs to Unix ACLs, the extended permissions provided by Windows/Mac ACLs will be removed from the existing files.

When you select Windows/Mac ACLs, the Mode will become greyed out as it only applies to Unix permissions. The default Windows ACLs are always set to what Windows sets on new files and directories by default. The Windows client should then be used to fine-tune the permissions as required.

8.3.10. Replacing a Failed Drive

If you are using any form of redundant RAID, you should replace a failed drive as soon as possible to repair the degraded state of the RAID. Depending upon the capability of your hardware, you may or may not need to reboot in order to replace the failed drive. AHCI capable hardware does not require a reboot.


a stripe (RAID0) does not provide redundancy. If you lose a disk in a stripe, you will need to recreate the volume and restore the data from backup.

Before physically removing the failed device, go to Storage –> Volumes –> View Volumes –> Volume Status and locate the failed disk. Once you have located the failed device in the GUI, perform the following steps:

  1. Click the disk’s entry then its “Offline” button in order to change that disk’s status to OFFLINE. This step is needed to properly remove the device from the ZFS pool and to prevent swap issues. Click the disk’s “Offline” button, pull the disk, then skip to step 3. If there is no “Offline” button but only a “Replace” button, then the disk is already offlined and you can safely skip this step.


if the process of changing the disk’s status to OFFLINE fails with a “disk offline failed - no valid replicas” message, you will need to scrub the ZFS volume first using its “Scrub Volume” button in Storage –> Volumes –> View Volumes. Once the scrub completes, try to Offline the disk again before proceeding.

  1. Once the disk is showing as OFFLINE, click the disk again and then click its “Replace” button. Select the replacement disk from the drop-down menu and click the “Replace Disk” button. If the disk is a member of an encrypted ZFS pool, you will be prompted to input the passphrase for the pool. Once you click the “Replace Disk” button, the ZFS pool will start to resilver. You can use the zpool status command in `Shell`_ to monitor the status of the resilvering.
  2. If the replaced disk continues to be listed after resilvering is complete, click its entry and use the “Detach” button to remove the disk from the list. Replacing a Failed Drive in an Encrypted Pool

If the ZFS pool is encrypted, additional steps are needed when replacing a failed drive.

First, make sure that a `passphrase has been set as described in `Creating an Encrypted Volume|outline>`_ before attempting to replace the failed drive. Then, follow the steps 1 and 2 as described above. During step 3, you will be prompted to input the passphrase for the pool. Wait until the resilvering is complete.

Next, restore the encryption keys to the pool. If the following additional steps are not performed before the next reboot, you may lose access to the pool permanently.

  1. Highlight the pool that contains the disk you just replaced and click the “Encryption Re-key” button in the GUI. You will need to enter the root password.
  2. Highlight the pool that contains the disk you just replaced and click the “Create Passphrase” button and enter the new passphrase. You can reuse the old passphrase if desired.
  3. Highlight the pool that contains the disk you just replaced and click the “Download Key” button in order to save the new encryption key. Since the old key will no longer function, any old keys can be safely discarded.
  4. Highlight the pool that contains the disk you just replaced and click the “Add Recovery Key” button in order to save the new recovery key. The old recovery key will no longer function, so it can be safely discarded. Removing a Log or Cache Device

If you have added any log or cache devices, these devices will also appear in Storage –> Volumes –> View Volumes –> Volume Status. If you click the device, you can either use its “Replace” button to replace the device as described above, or click its “Remove” button to remove the device.

Before performing either of these operations, verify the version of ZFS running on the system by running zpool upgrade -v|more from `Shell`_.

If the pool is running ZFSv15, and a non-mirrored log device fails, is replaced, or removed, the pool is unrecoverable and the pool must be recreated and the data restored from a backup. For other ZFS versions, removing or replacing the log device will lose any data in the device which had not yet been written. This is typically the last few seconds of writes.

Removing or replacing a cache device will not result in any data loss, but may have an impact on read performance until the device is replaced.

8.3.11. Replacing Drives to Grow a ZFS Pool

The recommended method for expanding the size of a ZFS pool is to pre-plan the number of disks in a vdev and to stripe additional vdevs using the `ZFS Volume Manager as described in `Extending a ZFS Volume`_ as additional capacity is needed.

However, this is not an option if you do not have open drive ports or the ability to add a SAS/SATA HBA card. In this case, you can replace one disk at a time with a larger disk, wait for the resilvering process to incorporate the new disk into the pool completes, then repeat with another disk until all of the disks have been replaced. This process is slow and places the system in a degraded state. Since a failure at this point could be disastrous, do not attempt this method unless the system has a reliable backup.

Check and verify that the autoexpand property is enabled before attempting to grow the pool. If it is not, the pool will not recognize that the disk capacity has increased. To verify the property, use `Shell`_. This example checks the ZFS volume named Vol1:

zpool get all Vol1
NAME   PROPERTY        VALUE                   SOURCE
Vol1   size            4.53T                   -
Vol1   capacity        31%                     -
Vol1   altroot         /mnt                    local
Vol1   health          ONLINE                  -
Vol1   guid            8068631824452460057     default
Vol1   version         28                      default
Vol1   bootfs          -                       default
Vol1   delegation      on                      default
Vol1   autoreplace     off                     default
Vol1   cachefile       /data/zfs/zpool.cache   local
Vol1   failmode        wait                    default
Vol1   listsnapshots   off                     default
Vol1   autoexpand      on                      local
Vol1   dedupditto      0                       default
Vol1   dedupratio      1.00x                   -
Vol1   free            3.12T                   -
Vol1   allocated       1.41T                   -
Vol1   readonly        off                     -
Vol1   comment         -                       default

If autoexpansion is not enabled, enable it by specifying the name of the ZFS volume:

zpool set autoexpand=on Vol1

Verify that autoexpand is now enabled by repeating zpool get all Vol1.

You are now ready to replace one drive with a larger drive using the instructions in Replacing a Failed Drive.

Replace one drive at a time and wait for the resilver process to complete on the replaced drive before replacing the next drive. Once all the drives are replaced and the resilver completes, you should see the added space in the pool.

You can view the status of the resilver process by running zpool status Vol1. Enabling ZFS Pool Expansion After Drive Replacement

It is recommended to enable the autoexpand property before you start replacing drives. If the property is not enabled before replacing some or all of the drives, extra configuration is needed to inform ZFS of the expanded capacity.

Verify that autoexpand is set as described in the previous section. Then, bring each of the drives back online with the following command, replacing the volume name and GPT ID for each disk in the ZFS pool:

zpool online -e Vol1 gptid/xxx

Online one drive at a time and check the status using the following example. If a drive starts to resilver, you need to wait for the resilver to complete before proceeding to online the next drive.

To find the GPT ID information for the drives, use zpool status [Pool_Name] which will also show you if any drives are failed or in the process of being resilvered:

zpool status Vol1
pool: Vol1
state: ONLINE
scan: scrub repaired 0 in 16h24m with 0 errors on Sun Mar 10 17:24:20 2013
NAME                                           STATE   READ WRITE CKSUM
Vol1                                           ONLINE  0    0     0
raidz1-0                                       ONLINE  0    0     0
gptid/d5ed48a4-634a-11e2-963c-00e081740bfe     ONLINE  0    0     0
gptid/03121538-62d9-11e2-99bd-00e081740bfe     ONLINE  0    0     0
gptid/252754e1-6266-11e2-8088-00e081740bfe     ONLINE  0    0     0
gptid/9092045a-601d-11e2-892e-00e081740bfe     ONLINE  0    0     0
gptid/670e35bc-5f9a-11e2-92ca-00e081740bfe     ONLINE  0    0     0
errors: No known data errors

After onlining all of the disks, type zpool status to see if the drives start to resilver. If this happens, wait for the resilvering process to complete.

Next, export and then import the pool:

zpool export Vol1

zpool import -R /mnt Vol1

Once the import completes, all of the drive space should be available. Verify that the increased size is recognized:

zpool list Vol1
Vol1   9.06T   1.41T   7.24T   31%     1.00x   ONLINE  /mnt

If you cannot see the extra space, you may need to run zpool online -e <pool> <device> for every device listed in zpool status.

8.3.12. Splitting a Mirrored ZFS Storage Pool

ZFSv28 provides the ability to to split a mirrored storage pool, which detaches a disk or disks in the original ZFS volume in order to create another identical ZFS volume on another system.


zpool split only works on mirrored ZFS volumes.

In this example, a ZFS mirror named test contains three drives:

zpool status
pool: test
state: ONLINE
scan: resilvered 568K in 0h0m with 0 errors on Wed Jul 6 16:10:58 2011
test           ONLINE  0    0     0
mirror-0       ONLINE  0    0     0
da1            ONLINE  0    0     0
da0            ONLINE  0    0     0
da4            ONLINE  0    0     0

The following command splits from the existing three disk mirror test a new ZFS volume named migrant containing one disk, da4. Disks da0 and da1 remain in test.:

zpool split test migrant da4

At this point, da4 can be physically removed and installed to a new system as the new pool is exported as it is created. Once physically installed, import the identical pool on the new system:

zpool import migrant

This makes the ZFS volume migrant available with a single disk. Be aware that properties come along with the clone, so the new pool will be mounted where the old pool was mounted if the mountpoint property was set on the original pool.

Verify the status of the new pool:

zpool status
pool: migrant
state: ONLINE
scan: resilvered 568K in 0h0m with 0 errors on Wed Jul 6 16:10:58 2011
migrant        ONLINE  0    0     0
da4            ONLINE  0    0     0
errors: No known data errors

On the original system, the status now looks like this:

zpool status
pool: test
state: ONLINE
scan: resilvered 568K in 0h0m with 0 errors on Wed Jul 6 16:10:58 2011
test           ONLINE  0    0     0
mirror-0       ONLINE  0    0     0
da1            ONLINE  0    0     0
da0            ONLINE  0    0     0
errors: No known data errors

At this point, it is recommended to add disks to create a full mirror set. This example adds two disks named da2 and da3:

zpool attach migrant da4 da2

zpool attach migrant da4 da3

The migrant volume now looks like this:

zpool status
pool: migrant
state: ONLINE
scan: resilvered 572K in 0h0m with 0 errors on Wed Jul 6 16:43:27 2011
migrant        ONLINE  0    0     0
mirror-0       ONLINE  0    0     0
da4            ONLINE  0    0     0
da2            ONLINE  0    0     0
da3            ONLINE  0    0     0

Now that the new system has been cloned, you can detach da4 and install it back to the original system. Before physically removing the disk, run this command on the new system:

zpool detach migrant da4

Once the disk is physically re-installed, run this command on the original system:

zpool attach orig da0 da4

Should you ever need to create a new clone, remember to remove the old clone first:

zpool destroy migrant

8.4. ZFS Scrubs

Storage –> ZFS Scrubs allows you to schedule and manage scrubs on a ZFS volume. Performing a ZFS scrub on a regular basis helps to identify data integrity problems, detects silent data corruptions caused by transient hardware issues, and provides early alerts to disk failures. If you have consumer-quality drives, consider a weekly scrubbing schedule. If you have datacenter-quality drives, consider a monthly scrubbing schedule.

Depending upon the amount of data, a scrub can take a long time. Scrubs are I/O intensive and can negatively impact performance. They should be scheduled for evenings or weekends to minimize the impact to users.

A ZFS scrub only checks used disk space. To check unused disk space, schedule S.M.A.R.T. Tests with a Type of Long Self-Test to run once or twice a month.

When you create a volume that is formatted with ZFS, a ZFS scrub is automatically scheduled for you. An entry of the same volume name is added to Storage –> ZFS Scrubs and a summary of this entry can be viewed in Storage –> ZFS Scrubs –> View ZFS Scrubs. Figure 8.4a displays the default settings for the volume named volume1. Table 8.4a summarizes the options in this screen.

Figure 8.4a: Viewing a Volume’s Default Scrub Settings


Table 8.4a: ZFS Scrub Options

You should review the default selections and, if necessary, modify them to meet the needs of your environment.

While a delete button is provided, deleting a scrub is not recommended as a scrub provides an early indication of disk issues that could lead to a disk failure. If you find that a scrub is too intensive for your hardware, consider disabling the scrub as a temporary measure until the hardware can be upgraded.

If you do delete a scrub, you can create a new scrub task by clicking Storage –> Volumes –> ZFS Scrubs –> Add ZFS Scrub.