Zvols
8 minute read.
The zvol screens and widgets, accessed from the Datasets screen, allow you to add or edit a zvol and manage the volume storage. Zvols are listed on the Datasets screen tree table.
The dataset tree table shows storage space used and available for that zvol (or dataset), encryption status (locked, unlocked, or unencrypted), and how that zvol or dataset is used (i.e., the system dataset, a share, virtual machine, or application).
Add Zvol shows on the Datasets screen when a dataset is selected, but does not show when a zvol is selected.
Each zvol has a set of Details for zvolname information cards (widgets) that provide information grouped by functional areas. Zvol widgets are:
The Encryption widget only shows if the zvol is encrypted.
The Zvol Details widget lists information on sync type, compression level, and ZFS deduplication settings. The Path shows the full path from a root (pool) dataset to the zvol location.
Edit opens the Edit Zvol screen for the selected zvol.
Delete opens the Delete zvol dialog.
The Delete Zvol dialog shows information about other options or services that use the zvol. It also shows the services that child datasets use. This includes information about snapshots, shares, or, if used, other services such as Kubernetes or VMs that use the dataset. Parent and child datasets include the Delete button.
The window includes a blank field where you type the path for the zvol. Confirm activates the Delete Zvol button.
The Zvol Space Management widget shows the space allocation (reserved, used, available) for the zvol.
When an encrypted zvol is locked, you must unlock it to see this widget.
The donut graph provides at-a-glance information and numeric values for the space allocated and used in the selected zvol.
This includes data written and space allocated to child datasets of this dataset.
The Encryption widget only shows when a zvol is configured with encryption. It shows the current state of the encryption, the encryption root, the type, and the algorithm used. The Encryption widget shows the Lock or Unlock options if it uses passphrase encryption. The Export Key option shows if the zvol uses key encryption.
Edit opens the Edit Encryption Options for zvol window for the selected zvol.
For more details on encryption windows and functions, see Encryption Settings.
The Data Protection widget displays for all datasets or zvols. It shows information for the number of snapshots and other data protection-related scheduled tasks (replication, cloud sync, rsync, and snapshots) configured on the system. It provides access to the tasks found on the Data Protection screen through links.
Take Snapshot opens the Add Snapshot screen.
View Snapshots opens the Snapshots screen list view where you can manage snapshots.
View Snapshot Tasks opens the Data Protection > Periodic Snapshot Tasks screen list view where you can manage scheduled periodic snapshot tasks.
Go To Backups opens the Data Protection screen, where you can create a data protection task like Rsync, Replication, or Cloud Sync Backup.
The Add Zvol and Edit Zvol screens allow admin users with the right permission level to create and modify zvols. Both screens include the same settings, but you cannot change the zvol name, Block Size, or select the Sparse option after you click Save on the Add Zvol screen. After adding a zvol, click Edit on the Zvol Details widget to open the Edit Zvol screen.
When the zvol is encrypted, Edit on the Encryption widget opens a configuration screen where you can change the passphrase for a zvol encrypted with the passphrase type, but you cannot change to a key encryption type. If the zvol is not encrypted, you do not see encryption options on the Edit Zvol screen.
| Setting | Description |
|---|---|
| Zvol name | (Required setting) Enter a short name for the zvol longer than 63 characters to prevent potential problems accessing zvols as devices. For example, you cannot use a zvol with a 70-character file name or path as an iSCSI extent. |
| Comments | Enter any notes about the zvol. Descriptions show on the iSCSI screens when the zvol is used in a block share. |
| Size for this zvol | Specify numeric size and value. You can include units like t as in TiB, and G. You can increase the size of the zvol later, but you cannot reduce the size. If the size is greater than 80% of the available capacity, the creation fails with an out-of-space error unless you select Force size. |
| Force size | Enables the system to create a zvol where the size is over 80% capacity. By default, the system does not create a zvol of this size. While not recommended, enabling this option forces the creation of the zvol. |
| Sparse | Enables using thin provisioning where disk space for this volume is allocated on-demand as new writes are received. Use caution when enabling, as writes can fail when the pool is low on space. |
| Sync | Select a data write synchronization option from the dropdown list. Options: |
| Compression level | Select the option from the dropdown list for the type of data compression to use for encoding information in less space than the original data occupies. Select the algorithm that balances disk performance with the amount of space saved. See below for the options. |
| ZFS Deduplication | Do not change this setting unless instructed to by your TrueNAS support engineer. Transparently reuses a single copy of duplicated data to save space. Deduplication can improve storage capacity, but it is RAM-intensive. Compressing data is generally recommended before using deduplication. Deduplicating data is a one-way process. Deduplicated data cannot be undeduplicated!. |
| Read-only | Select the option to prevent modifying the zvol. Options are Inherit (off), On or Off. |
| Block size | Select the size option from the dropdown list. The default is 16KiBt, other options are 4KiB, 8KiB, 16KiB, 32KiB, 64KiB, 128KiB. The zvol default block size is automatically chosen based on the number of disks in the pool for a general use case. |
| Snapdev | Select the option that controls whether the volume snapshot devices under /dev/zvol/poolname are hidden or visible from the dropdown list. Options are Inherit (hidden), Visible and Hidden (default value). |
| Inherit (non-encrypted/encrypted) | Encryption settings are inherited from the parent dataset. When the parent is encrypted, this option defaults to Inherit (encrypted); otherwise, it shows (non-encrypted). Clearing the checkmark shows the Encryption options. If the parent is encrypted with the passphrase type, the zvol can only use passphrase encryption. When the parent is encrypted with a key, the zvol can use either key or passphrase encryption. Refer to the Encryption Settings article for more details. |
Depending on their workload, zvols can require additional tuning for optimal performance. See the OpenZFS handbook workload tuning chapter for more information.
The Compression Level setting lists several compression algorithm to choose from. Select the option that best suits your needs from the dropdown list.
LZ4 maximizes performance and dynamically identifies the best files to compress. LZ4 provides lightning-fast compression/decompression speeds and comes coupled with a high-speed decoder. This makes it one of the best Linux compression tools for enterprise customers.
ZSTD offers highly configurable compression speeds, with a very fast decoder.
Gzip is a standard UNIX compression tool widely used for Linux. It is compatible with every GNU software which makes it a good tool for remote engineers and seasoned Linux users. It offers the maximum compression with the greatest performance impact. The higher the compression level implemented the greater the impact on CPU usage levels. Use with caution especially at higher levels.
ZLE or Zero Length Encoding, leaves normal data alone but only compresses continuous runs of zeros.
LZJB compresses crash dumps and data in ZFS. LZJB is optimized for performance while providing decent compression. LZ4 compresses roughly 50% faster than LZJB when operating on compressible data, and is greater than three times faster for uncompressible data. LZJB was the original algorithm used by ZFS but it is now deprecated.