Monitoring zfs arc size (for tuning)

[Johannes Totz]

Heya,

I've been monitoring my arc size, hit and miss rates etc to determine
what a minimum arc size would be without performance going down the
drain, for my workload.

The only interesting point so far is that arc size still exceeds its
limit occasionally!
That is, kstat.zfs.misc.arcstats.size is larger than
kstat.zfs.misc.arcstats.c_max (which is the same as vfs.zfs.arc_max) by
a few hundred megabytes.

Also, some of the periodic tasks that this machine does makes
kstat.zfs.misc.arcstats.size go down to almost zero, and then shoot up
to the max again in a short period of time.


This is on:

FreeBSD XXX 8.2-STABLE FreeBSD 8.2-STABLE #0 r224227: Wed Jul 20
16:55:23 BST 2011     root at XXX:/usr/obj/usr/src/sys/GENERIC  amd64



Johannes