I've been having problems with horrible performance on my Mac desktop.  Constant grinding of the hard driver, non-stop lagging apps.  Turns out MDS (the spotlight process) seemed to be constantly running.  Google brought some suggestions such as completely rebuilding the spotlight index, but this ultimately didn't work.  Even turning spotlight off seemed to miss the mark. Finally, I made two changes which solved the problem completely:

• I added /Library/Backblaze to the Spotlight exclusions list.
• I forced a Time Machine backup, then added that volume to the exclusions list.

That's it!  A simple fix restored the performance of my machine.  I'm unsure as to why it started happening all of a sudden, but I'll take it.Up Update 2/5/2011: A clue to what might be going on as evidenced by the following command:

• sudo fs_usage -w -f filesys mdworker

This command shows exactly what Spotlight is doing, and essentially it appears that whenever a file is accessed on the filesystem, Spotlight gives it a look for indexing.  This means when you're backing up (either via Backblaze or Time Machine) lots of files are getting "touched" and thus need to be examined by Spotlight.  This impacts an already I/O intensive process, bringing the system to a crawl.  Since this post, I've had zero performance problems and I'm going to consider this one solved.  Also, on my MacBook Air, Backblaze had already added itself to the exclusions list, so it's possible that this has been addressed when you do a fresh install of Backblaze nowadays.