Late in the day yesterday (~4:00pm) the disks attached to our email server had a hiccup which caused a mirror system to fail. Thus the server needed to repair the mirror. Over the next several hours email was extremely slow. Users with small mailboxes (a few megs or less) were able to use email. Everyone else had major issues. Some of the resyncing finished over night while the system was relatively quiet. Then this morning things slowed down again as people stated to check their email. So inorder to get the resyncing to finish in a timely manner, we stopped all IMAP and POP connections to the mailserver at about 9:55am. By 10:15am it had finished and email was back to “normal” speed.
So, lessons learned. Hot swappable is not always the case. Large inboxes are slow and prone to performance issues. We need more/faster disks to handle the load.