ZFS cache and log

There are two kinds of cache, read cache and write cache.

Read cache

Called ARC and L2ARC.

ARC (Adaptive Replacement Cache)

In memory, caching the information that would require in the near future, while discarding the ones that will be needed furthest ahead in time.

This can be set using kernel/module parameter, such as zfs_arc_max.

L2ARC (Level 2 ARC)

In cache device, extension of ARC. Can be created using following command

zpool add tank cache ada3

Note: tank is the pool name, ada3 is the block device used for caching

Write cache

Called ZIL (ZFS Intent Log).

Asynchronous

By default, ZFS will cache write data in memory before write to disk, this is called asynchronous mode.

Synchronous

Synchronous will make sure data written to disk before continue, this can be set using following command

zfs set sync=always mypool/dataset1

ZFS Intent Log (ZIL)

This is the temporary space to store data before write into main disks, this can be used to speed up write operation. The write operation is considered as completed once data written into ZIL device, which is called SLOG (Separate Intent Log) devices, can be defined as follow

zpool add tank log ada3

Note: tank is the pool name, ada3 is the block device used for slog

If worrying SLOG device faulty, it can be mirrored too.

zpool add tank log mirror ada3 ada4

References

Configuring ZFS Cache for High Speed IO
ZFS Performance with Databases (Cached)