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SLOG Overprovisioning

  2 minute read.

Last Modified 2022-05-18 14:48 EDT

Over-provisioning SLOG SSDs is useful for different scenarios. The most useful benefit of over-provisioning is greatly extending SSD life. Over-provisioning an SSD distributes the total number of writes and erases across more flash blocks on the drive.

Seagate provides a thoughtful investigation into over-provisioning SSDs here:

Some SATA devices are limited to one resize per power cycle. Some BIOS can block resize during boot and require a live power cycle.

Overprovision Options

To over provision a SLOG device, log in to TrueNAS and go to System > Advanced. Enter an over-provision value corresponding to the new size in GB in the Log (Write Cache) Overprovision Size in GiB field.


When this value is applied, the over-provision value is applied whenever a pool is created with a SLOG device. It is impossible to restore an over-provisioned SLOG device back to original capacity without running command disk_resize after first destroying the pool it was part of and issuing a full power cycle.

Only one over-provision/under-provision operation occurs per power cycle.

Erasing the over-provision setting in System > Advanced Log (Write Cache) Overprovision Size in GiB field and setting to none prevents future SLOG devices from being over-provisioned.

Use disk_resize in the Shell to over-provision.

The command to over-provision an SSD is disk_resize {DEVICE} {SIZE}, where {DEVICE} is the SSD device name and {SIZE} is the new provision size in GiB or TiB. Example: disk_resize ada5 16GB. When no size is specified, it reverts the provision back the full size of the device.