Pause while selecting INBOX

[Rob Mueller]

> 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.

Some more information about your setup would be helpful.

1. What OS? 32 or 64bit?
2. How much RAM do you have? How much is used by cache?
3. What sort of drive system is the data stored on?
4. Do you have split data/meta data?
5. What filesystem are you using?
6. What mount options to you have on your volumnes?
7. What IO scheduler are you using?
8. What version of cyrus are you using?
9. Do you have the cyrus status cache enabled?

Some things to try if you haven't:

1. Make sure you have noatime (and nodiratime if applicable) set on your FS
2. If on linux, use the deadline IO scheduler
3. If you can, split your data and meta data onto separate volumes (probably 
a lot of work)
4. Install more RAM and use a 64-bit OS. x64_64 linux can cache many, many 
more inodes and dentries in memory than x86 linux.
5. Upgrade to the latest cyrus and enable the status cache

Rob