Mirroring the Boot Pool

How to add a second drive and mirror the system boot pool.

  2 minute read

TrueNAS supports mirroring a boot pool by adding a second device to the original pool. This is especially helpful as one device can fail and the system will still boot. If a device were to fail in a mirror, it can be easily detached and replaced with a new device.

View the current boot pool status by going to System > Boot > ACTIONS > Boot Pool Status. On this page you will find the list and status of each boot pool on the system. The default boot pool is named truenas-boot. If adding a second device to create a mirrored boot pool, consider these caveats:

  • The new device must have at least the same capacity as the existing device. Larger capacity devices can be added, but the mirror will only have the capacity of the smallest device. Different models of devices which advertise the same nominal size are not necessarily the same actual size. For this reason, adding another device of the same model of is recommended.

  • It is strongly recommended to use SSDs rather than USB devices when creating a mirrored boot pool.



Click   on a device entry to access actions specific to that device:

  • Attach: Use this option to add a second device to create a mirrored boot pool. If another device is available, it appears in the Member Disk drop-down menu. Select the desired device. The Use all disk space option controls the capacity made available to the device. By default, the new device is partitioned to the same size as the existing device. When Use all disk space is enabled, the entire capacity of the new device is used. If the original operating system device fails and is detached, the boot mirror will consist of just the newer device, and will grow to whatever capacity it provides. However, new devices added to this mirror must now be as large as the new capacity. Click SAVE to attach the new device to the mirror.

  • Detach: remove the failed device from the mirror so that it can be replaced.

  • Replace: once the failed device has been detached, select the new replacement device from the Member Disk drop-down menu to rebuild the mirror.


Last modified August 31, 2020: Update mirror-boot-pool.md (51fe20c0)