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  9 minute read.

Last Modified 2022-06-01 15:15 EDT

TrueNAS uses ZFS data storage “pools” to efficiently store and protect data.

Storage pools are attached drives organized into virtual devices (vdevs). ZFS and TrueNAS periodically review and heal when discovering a bad block in a pool. Drives are arranged inside vdevs to provide varying amounts of redundancy and performance. Combined, ZFS and vdevs combined create high-performance pools, pools that maximize data lifetime, and all situations in between.

Review Storage Needs

We strongly recommend users review the available system resources and plan the storage use case before creating a storage pool.

  • Allocating more drives to a pool increases redundancy when storing critical information.
  • Maximizing total available storage at the expense of redundancy or performance entails allocating large-volume disks and configuring a pool for minimal redundancy.
  • Maximizing pool performance entails installing and allocating high-speed SSD drives to a pool.

Determining your specific storage requirements is a critical step before creating a pool.

Creating or Importing a Pool

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To create a new pool, go to Storage and click Create Pool.


First, enter a pool name.

TrueNAS offers several encryption algorithms to maximize security. However, encryption also complicates data retrieval and risks permanent data loss! Refer to the Encryption article for more details and decide if encryption is necessary for your use case before setting any Encryption option.

Next, configure the virtual devices (vdevs) that make up the pool.

If the disks used have non-unique serial numbers, they do not populate the Available Disks section until the Show disk with non-unique serial numbers checkbox is selected.

Suggested Layout

Clicking Suggest Layout allows TrueNAS to review all available disks and populate the primary Data VDevs with identically sized drives in a configuration balanced between storage capacity and data redundancy. Click Reset Layout to clear the suggestion.

To manually configure the pool, add vdevs according to your use case. Check the Disk checkboxes and click the   to move the disks into a vdev.

Warning: USB-connected disks may report their serial numbers inaccurately, making them indistinguishable from each other.

Vdev Types

Pools offer several vdev types. Vdevs store data or enable unique features for the pool.

To add a vdev type during pool creation, click Add Vdev() and select the type. Select disks from Available Disks and use the   (right arrow) next to the new VDev to add it to that section.

Data Type

Data is the standard vdev for primary storage operations. Each storage pool requires at least one Data vdev. Data vdev configuration typically affects how users can configure other types of vdevs.

Users can duplicate a Data VDev by clicking Repeat. When the system has more available equal-sized disks, the Repeat button creates another vdev with an identical configuration called a Mirror.


When even more same-size disks are available, users can create multiple copies of the original vdev.

We do not recommend having multiple data vdevs with different numbers of disks in each vdev. Doing so complicates and limits pool capabilities.

Additional Types

  • Cache: ZFS L2ARC read-cache used with fast devices to accelerate read operations. Users can add or remove Cache VDevs after creating the pool.
  • Log: ZFS LOG device that improves synchronous write speeds. Users can add or remove Log VDevs after creating the pool.
  • Hot Spare: Drives reserved for inserting into Data vdevs when an active drive fails. The system uses hot spares as temporary replacements for failed drives to prevent larger pool and data loss scenarios. When a user replaces a failed drive with a new one, the hot spare reverts to an inactive state and becomes available again as a hot spare. If a user detaches the failed drive from the pool without adding a new one, the system promotes the temporary hot spare to a full Data vdev member.
  • Metadata: Special allocation class used to create Fusion Pools for increased metadata and small block I/O performance.
  • Dedup: Stores ZFS de-duplication. Requires allocating X GiB for every X TiB of general storage. Example: 1 GiB of Dedup vdev capacity for every 1 TiB of Data vdev availability.

Vdev Layouts

Disks added to a vdev arrange in different layouts, according to the specific pool use case.

We do not recommend mixing disks of different sizes in a vdev. If you do, you must Force the action and override the One or more data vdevs has disks of different sizes error.



The Pool Manager suggests a vdev layout from the number of disks added to the vdev. For example, if you add two disks, TrueNAS automatically configures the vdev as a Mirror. The total available storage is the size of one added disk while the other disk provides redundancy.


To change the vdev layout, open the Data VDevs list and select the desired layout.

TrueNAS SCALE does not support adding multiple vdevs with different layouts to a pool. Create a new pool when a different vdev layout is required. For example, pool1 has a data vdev in a mirror layout, so create pool2 for any raid-z vdevs.
  • Stripe: Each disk stores data. A Stripe requires at least one disk and has no data redundancy.
  • Mirror: Data is identical in each disk. A Mirror requires at least two disks, provides the most redundancy, and has the least capacity.
  • RAIDZ1: Uses one disk for parity while all other disks store data. RAIDZ1 requires at least three disks.
  • RAIDZ2: Uses two disks for parity while all other disks store data. RAIDZ2 requires at least four disks.
  • RAIDZ3: Uses three disks for parity while all other disks store data. RAIDZ3 requires at least five disks.
Never use Stripe to store critical data! A single disk failure results in losing all data in the vdev.
The import procedure only applies to disks with a ZFS storage pool. To import disks with different file systems, see the SCALE Disks article.

ZFS pool importing works for pools that were exported or disconnected from the current system, created on another system, and pools to reconnect after reinstalling or upgrading the TrueNAS system.

When physically installing ZFS pool disks from another system, use the zpool export poolname command in the command line or a web interface equivalent to export the pool on that system. Shut that system down and move the drives to the TrueNAS system. Shutting down the original system prevents an in use by another machine error during the TrueNAS import.

To import a pool, go to Storage and click Import.

TrueNAS detects any pools that are present but unconnected.

Select a pool from the Pool drop-down and click Next.


Review the Pool Import Summary and click Import.


Since GELI encryption is specific to FreeBSD, TrueNAS SCALE cannot import GELI-encrypted pools. See the Migrating GELI-encrypted Pools to SCALE section in the Installing SCALE article.

Pool Operations

Use the Pool Operations button to manage a pool.


Pool Actions


Pool Options allows users to set/unset Auto TRIM.

With Auto TRIM selected and active, TrueNAS periodically checks the pool disks for storage blocks it can reclaim. Auto TRIM can impact pool performance, so it’s disabled by default.

For more details about TRIM in ZFS, see the autotrim property description in zpool.8.


The Export/Disconnect option disconnects the pool to transfer drives to a new system for importing or completely deletes the pool and any data stored on it.

A dialog box displays any system services affected by exporting the pool.

Users can erase all data on the pool by checking the Destroy data on this pool? box.

Clicking the Delete configuration of shares that used this pool? box deletes shares connected to the pool.

The Add Vdevs button opens the Pool Manager so users can add Vdevs to the pool.

Users cannot change the original encryption or data Vdev configuration.

TrueNAS selects Data VDev by default. To add different Vdev types to the pool, select one from the Add Vdev drop-down.

When adding disks to increase the capacity of a pool, ZFS supports adding Vdevs (virtual devices) to an existing ZFS pool. After creating a Vdev, you cannot add more drives to that Vdev. However, you can stripe a new Vdev with another of the same type to increase the overall pool size To extend a pool, you must add a Vdev that is the same type as existing Vdevs.

Vdevs extending examples:

  • To make a striped mirror, add the same number of drives to extend a ZFS mirror. For instance, if ten new drives are available, you could initially create a mirror of two drives, then extend the mirror by adding another mirror of two drives, repeating three more times until you’ve added all ten drives.
  • To make a stripe of two RAIDZ1 Vdevs (similar to RAID 50 on a hardware controller), add another three drives to extend a three-drive RAIDZ1.
  • To make a stripe of RAIDZ2 vdevs (similar to RAID 60 on a hardware controller), add another four drives to extend a four-drive RAIDZ2.
  • Add a disk as a Hot Spare to the pool.


Scrub Pool initiates a pool data integrity check.

If TrueNAS detects any problems during the scrub, it either corrects them automatically or generates an alert in the web interface.

By default, TrueNAS automatically checks every pool is on a reoccurring scrub schedule.


Status displays the state of the last scrub and disks in the pool.

The Pool Status screen has additional disk management options.

Expand Pool increases the pool size to match all available disk space. A user typically expands a pool when virtual disks are resized apart from TrueNAS.

The Upgrade Pool option only appears when TrueNAS can upgrade the pool to use new ZFS feature flags.

Before upgrading an existing pool, be aware of these caveats:

  • Upgrading a pool is one-way. You cannot regress to an earlier ZFS version or downgrade to an earlier software version that does not support those ZFS features.
  • Upgrading a pool unlikely affects data, but we recommend backing up data for safety. Before performing any operation that can affect the data on a storage disk, always back up all data first and verify the backup’s integrity.
  • Upgrading a ZFS pool is optional. Do not upgrade the pool if you may revert to an earlier TrueNAS version or repurpose the disks in another OS that supports ZFS. Upgrading a pool is unnecessary unless the end-user specifically needs the newer ZFS Feature Flags. If you upgrade a pool to the latest feature flags, you cannot import it into another OS that does not yet support them.

Upgrading a pool only takes a few seconds and is non-disruptive. You do not need to stop any sharing services to upgrade a pool. However, we recommend upgrading when the pool is not seeing heavy use. Upgrading suspends I/O for a short period, but is nearly instantaneous on a quiet pool.

Encryption Actions

See the SCALE Encryption page for detailed encryption information.