LARGE single-system Cyrus installs?

Michael Bacon baconm at
Thu Nov 15 10:04:34 EST 2007

Interesting thought.  We haven't gone to ZFS yet, although I like the idea 
a lot.  My hunch is it's an enormous win for the mailbox partitions, but 
perhaps it's not a good thing for the meta partition.  I'll have to let 
someone else who knows more about ZFS and write speeds vs. read speeds 
chime in here.

I have heard tell of funny behavior that ZFS does if you've got 
battery-backed write caches on your arrays.  (i.e., ZFS aggressively 
flushes those caches, making them essentially useless.)  Here's the best 
article I was able to find on it in a short google:

Only do that, of course, if you can trust your drives to write the data in 
the caches in case of a power outage, and if you don't have any non-battery 
backed caches somewhere else on your system.


--On Wednesday, November 14, 2007 8:15 PM -0800 Vincent Fox 
<vbfox at> wrote:

> This thought has occurred to me:
> ZFS prefers reads over writes in it's scheduling.
> I think you can see where I'm going with this.  My WAG is something
> related to Pascal's, namely latency.  What if my write requests to
> mailboxes.db
> or deliver.db start getting stacked up, due to the favoritism shown to
> reads?
> The actual usage mix on a Cyrus system ends up being more writes than
> reads. Despite having channels that seem under-utilized perhaps the
> stacking of small latencies hits a tipping point that is causing the
> slowdown.
> We have all Cyrus lumped in one ZFS pool, with separate filesystems for
> imap, mail, sieve, etc.  However, I do have an unused disk in each array
> such that I could setup a simple ZFS mirror pair for /var/cyrus/imap so
> that the databases are in their own pools.  Or even I suppose a UFS
> filesystem with directio and all that jazz set.  Perhaps I wouldn't get
> all the
> effects of a RAM-SAN 500 but it could be a worthwhile improvement.
> I liked having one big pool, but if it works, c'est la vie.
> ----
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