What is the optimal cache size for ZFS drives?

[wayner]

It seems like unRAID uses 1/8 of your RAM as the size of the cache for ZFS disks.  You can change this, but not currently through the GUI - you add a config file.

 

Has anyone done research into determining the optimal cache size?  I recently increased my server RAM from 32GB to 64TB and my ZFS cache size went up to 8GB.  But I rarely use all of my RAM.  Should I increase the ZFS cache size?  In what instances does it make sense to have a larger or smaller cache size?

 

I did search but didn't seem any documentation on this.  There is a thread that is 1.5 years ago, but it is unclear to me whether this all applies as this predates unRAID official support for ZFS.  

[wayner]

No one has any thoughts on this?

 

Is there any reason why you can't set the ZFS cache size in the unRAID GUI?  Is it because it can only be resized on a reboot?  Or it is a new feature an just hasn't yet been implemented?

[JorgeB]

You can Goole zfs ARC for more info, there's nothing special about zfs on Unraid, personally I use as much RAM as I can spare for it in each server, but note that ARC won't help everything, it helps mostly with metadata and if you keep accessing the same files.

[wayner]

Thanks.  One of my main uses of unRAID is as a media server which will be accessing lots of metadata and Fanart files.

 

Is there more documentation around pros and cons of various disk formats?  Like what format should you use for your cache disk?  BTRFS, ZFS, XFS?  What are the pros and cons of each?

[JorgeB]

Nothing Unraid specific, but if you google that it should be easy to find, possibly will also find older discussions on this forum.