ZFS RaidZ2 with 24 drives?

[James R. Van Artsdalen]

Solon Lutz wrote:
> I opted for two 12-disc raidz2. 
> Reasons were: Space is more important than performance.
>
> But performance is very poor - have a look at the iostats

Try a RAIDZ with fewer drives - maybe four drives - and see if
performance is better.

12 drives in RAIDZ2 may result in a stripe that is just too big for the
writes your tests do.

> sometimes nothing really seems
> to happen for up to ten seconds, or very little data gets  written. Might this be a problem
> of the amd64 system having only 4GB of RAM? Any tuneable sysctls? Enabling prefetch didn't help...:

Any copy-on-write filesystem is going to greatly benefit from very
aggressive write-deferral and combining.  The cache probably isn't
flushed unless it needs to be, or until the next transaction group
commit.  "10 seconds" is roughly the usual interval for ZFS commits.

This is likely not a problem.