Bad performance when accessing a lot of small files

[dima]

>> Increase the dirhash maxmem
>> I have 'sysctl -w vfs.ufs.dirhash_maxmem=268435456' (256Mb)in my
>> rc.local on a (maildir) fileserver here
>>
> This is a zfs filesystem, by the way I raise this limit as suggested.
> 
> I let you know

Can you provide
sysctl vfs.zfs output then?

ZFS can be tuned by increasing kern.maxvnodes sysctl (don't forget to increase vm.kmem_size and vm.kmem_size_max accordingly). Do it until vfs.numvnodes would stabilize below the kern.maxvnodes value during peak load patterns.