Just in case it is of general interest: ZFS mirroring was the culprit in our case

[Vincent Fox]

Can you expand on this, like a LOT?

I recall a while ago you brought up some performance issues and
said you had found hacks for them.  Were those issues actually unresolved
or are you talking about something else?  I don't see any recent posts by
you about problems with your Cyrus install.

I'm struggling to see the mechanism by which mirroring creates a problem.
Were you resilvering at the time?

Pascal Gienger wrote:
> Our latency problems went away like a miracle when we detached one half of 
> the mirror (so it is no more a mirror).
>
> Read-Rates are doubled (not per device, the total read rate!), latency is 
> cut off. No more latency problems.
>
> When attaching the volume again, resilvering puts the system to a halt - 
> reads and writes do block for seconds (!). We will go on directly with Sun 
> to solve the problem. Their "lowest I/O-priority to resilver disks" does 
> not seem to be effective. It really blocks the kernel and you end up with 
> thousand locks in "zfs_zget".
>
> We have two SAN volumes in different buildings which are NOT the 
> bottleneck, tests show it.
>
> Pascal
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