read/write cache drive (s) setup

[tralfaz]

hey folks,
can someone direct me to instructions on using drives 6 and 7 in my truenas mini as write and read cache drives. i have a pair of 1tb ssd's just for this purpose but have reached a dead end as far as the web interface goes. thank you.

 

[jgreco]

There is no "write cache drive" in ZFS. ZFS uses main system memory for write cache. Some people incorrectly refer to a SLOG device as write cache, but it is not. See

Some insights into SLOG/ZIL with ZFS on FreeNAS

What is the ZIL? POSIX provides a facility for the system or an application to make sure that data requested to be written is actually committed to stable storage: a synchronous write request. Upon completion of a sync write request, the underlying filesystem is supposed to guarantee that a...

www.truenas.com

It is super unlikely that a random 1TB SSD would make a good SLOG device, because you need power loss protection in the SSD, which most do not have. However, it is also likely that you don't need a SLOG device.

As for read cache, it is not recommended to exceed 10x the size of system RAM for L2ARC, and it is suggested to start at 5x. As far as I'm aware, the Mini' typically comes with 16-64GB of RAM. Forum experience is that RAM amounts less than 64GB may not make productive and good use of the L2ARC, and 64GB x 10 = 640GB, so the maximum SSD size recommended would probably be 500GB. You might be able to use the 1TB SSD's for storage for VM's, jails, or containers though.

 

[tralfaz]

thank you for that. it all sounds very reasonable. i think my confusion here stems from the truenas product build page which offers to set up one ssd as a write cache and one as a read cache. i must be misunderstanding what they are offering here. i populated the drives myself rather than having ixsystems prepopulate the drives. they specifically offer what they call accelerator packs that consist of 480GB SSD's for read and write caching.

 

[jgreco]

It's quite possible that whatever they preinstall is suitable for use as SLOG. I don't know since I've never bought one. SSD prices are somewhat crazy low right now.