ZFS read/write performance slows with time

[andy thomas]

A server running 64-bit FreeBSD 8.0 boots from a SATA disk and then mounts 
a ZFS mirror consisting of two SAS disks plus one spare. Immediately after 
booting, the filesystem is fast and responsive and 'zpool iostat -v tank' 
reports read and write disk bandwidths of over 22 MB.

But over a period of time, this performance begins to deteriorate and 
after 180 days of uptime this server, which is running mail, samba and 
webmail servers in 3 separate jails, really struggles especially the IMAP 
daemon. zpool iostat -v reports a maximum read bandwidth of around 2MB and 
a write bandwidth of 143 KB maximum. Rebooting the system restores normal 
performance but the cycle gradually repeats itself.

I can't see anything wrong in any log and the system has 12 GB of memory 
and a 2 Ghz quad-core Xeon CPU so it isn't under-resourced. At boot time 
ZFS reports its version as being 13 - could the problem be due to a memory 
leak or some other issue with early versions of ZFS that have since been 
fixed in later FreeBSD releases?

Andy