some general zfs tuning (for iSCSI)

[Eugene M. Zheganin]

Hi,


I'm using several FreeBSD zfs installations as the iSCSI production 
systems, they basically consist of an LSI HBA, and a JBOD with a bunch 
of SSD disks (12-24, Intel, Toshiba or Sandisk (avoid Sandisks btw)). 
And I observe a problem very often: gstat shows 20-30% of disk load, but 
the system reacts very slowly: cloning a dataset takes 10 seconds, 
similar operations aren't lightspeeding too. To my knowledge, until the 
disks are 90-100% busy, this shouldn't happen. My systems are equipped 
with 32-64 gigs of RAM, and the only tuning I use is limiting the ARC 
size (in a very tender manner - at least to 16 gigs) and playing with 
TRIM. The number of datasets is high enough - hundreds of clones, dozens 
of snapshots, most of teh data ovjects are zvols. Pools aren't 
overfilled, most are filled up to 60-70% (no questions about low space 
pools, but even in this case the situation is clearer - %busy goes up in 
the sky).

So, my question is - is there some obvious zfs tuning not mentioned in 
the Handbook ? On the other side - handbook isn't much clear on how to 
tune zfs, it's written mostly in the manner of "these are sysctl iods 
you can play with". Of course I have seen several ZFS tuning guides. 
Like Opensolaris one, but they are mostly file- and 
application-specific. Is there some special approach to tune ZFS in the 
environment with loads of disks ? I don't know.... like tuning the vdev 
cache or something simllar. ?


Thanks.

Eugene.