MTMFS consuming an entire CPU core and fans running full bore with Lion
Solution 1:
To track down the issue you may have to dive into fs_usage or other tools that capture filesystem activity as it happens.
Normally, I have seen minor corruption issues on a filesystem (or bugs in the code - it's hard to tell / hard to reproduce this issue) as the cause of this. In practice I can't get it to fail again after cleaning the existing local backup.
sudo tmutil disablelocal- reboot the Mac in safe mode (which runs fsck and many other helpful and potentially irrelevant tests and checks for corruption)
- reboot the mac normally
sudo tmutil enablelocal
Solution 2:
I suffered this today when doing a find / -whatever -exec this-or-that {} \;
Suddenly mtmfs got 100% CPU. It turns out that mtmfs is a special filesystem mounted on /Volumes/MobileBackups. If you run something that will access files indiscrimately, such as a find (something many "cleaner" programs do) mtmfs will use a lot of CPU when its files are being accessed.
You have two options:
1) As suggested, disable local Time Machine backups (sudo tmutil disablelocal)
2) Make sure to exclude /Volumes/MobileBackups (or even /Volumes, as it can be a p.i.t.a. to run a find over network file systems) from the search command.
That should solve the issue.
Solution 3:
Running sudo find / -type f -mmin -5 while mtmfs was using 85% of my Mac, revealed that the process was writing to /Volumes/MobileBackups/Backups.backupdb.
I guess that TM was doing its daily local backup.