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SLOG Overprovisioning

  2 minute read.

Last Modified 2021-02-25 12:45 EST

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

Seagate provdes a thoughtful investigation into overprovisioning SSDs: https://www.seagate.com/tech-insights/ssd-over-provisioning-benefits-master-ti/.

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

To over provision a SLOG device, log in to TrueNAS and go to System > Advanced. Enter an overprovision value corresponding to the new size in GB.

StoragePoolsOptionSLOGOverprovision

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

Only one overprovision/underprovision operation occurs per power cycle.

Erasing the overprovision setting and setting to none prevents future SLOG devices from being overprovisioned.

Use disk_resize in the Shell to overprovision.

The command to overprovision 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.