change in io statistics since updating 8.2-STABLE

[Robert Schulze]

Hello,

like described in another thread, we recently updated a fileserver from 
8.2-STABLE at 2011-05-08 to 8.2-STABLE at 2011-09-08 (amd64 both). ZPool is 
left at v14, the filesystems at version 3. Nothing in /boot/loader.conf 
or /etc/sysctl.conf has been changed.

We now realize that r/w behaviour has changed on the whole pool.
The l2arc devices (2 SSDs) have decreased in writes from average 10M/sec 
(peaks at 20 M/sec) to <5M/sec.

On the other hand, reads on the disks (2x5 disks raidz2) have increased 
(nearly doubled on each disk).

I suspect, that the cache is somewhat faulty or has become slow, and so 
ZFS decides to read data from main storage instead of the cache. Is 
there another explaination? Are there some defaults of sysctls which 
have been changed from May to September?

with kind regards,
Robert Schulze