Speeding up the Time Machine

I've been wondering recently why Time Machine has been taking so long to do its hourly backups. For the most part, not a lot changes on this machine, yet the backups often take up to 20 mins (including the pre- and post-processing). This can slow the machine down a bit as both the internal and firewire disks grind away.

After a bit of investigation, I discovered a blog post which said that large mailboxes in Mail can slow down Time Machine. Aha! That makes sense - Apple switched to using a maildir-like format which means that all emails are stored as individual files. While this makes sense from an application performance perspective, it really slows down backups as it looks at each file. Being subscribed to quite a few mailing lists (including most of the major FreeBSD lists), I receive quite a lot of mail, so my mail folders are all very large - tens of thousands of messages in each folder - plus the main archive of my Inbox.

I store all of my mail on my IMAP server, so I decided to tell Time Machine not to bother backing up ~/Library/Mail anymore - et voila! Backups are now much faster - usually only a couple of minutes, or up to 10 when it's purging expired backups. This is much more reasonable.