Pause while selecting INBOX

[Rob Banz]

On Sep 26, 2008, at 06:44, Gary Mills wrote:

> We have a moderately sized Cyrus installation with 2 TB of storage
> and a few thousand simultaneous IMAP sessions.  When one of the
> backup processes is running during the day, there's a noticable
> slowdown in IMAP client performance.  When I start my `mutt' mail
> reader, it pauses for several seconds at `Selecting INBOX'.  That
> behavior disappears when the backup finishes.
>
> Where's the first place to look for this problem?  I/O statistics
> show a higher read bandwidth while the backup is running, but writes
> still dominate.  The backup would typically read all of the files in
> a single Cyrus partition.

As always, the answer is probably complicated.

What's your storage backend look like? Some sort of RAID with cache?  
The backup is very probably blowing out your cache while running,  
which can manifest itself in ways that will make it look like the  
performance profile of your storage has taken on a different  
personality. Especially with respect to writes, since you might  
usually be blessed with an abundance of write-back cache that's now  
fighting with all the reads for the backup. Do you have your cyrus  
partitions mounted with noatime? Your backups could be causing fs  
write operations that you really don't want.

-rob