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Time Machine Startup Beach Ball Hang


2013/10/06

I had a problem with Time Machine Backups hanging up my machine at the beginning of a backup. The hang was not long; I'd just get spinning beach balls while the backup disk spun up. After that, I'd have normal operations again until the next time that disk was called for backup.

I use two Time Machine disks - Time Machine switches between them automatically, first using one and then the other. Only one of the disks caused the beach ball hang.

Plainly there is something different about this disk. It's a sleepy head, it doesn't like waking up. Plainly there is something wrong with Time Machine also as it shouldn't usurp other CPU scheduling so aggressively.

I Googled around a bit and found a bunch of things that sound to me like OPPs (Other People's Problems) so I did a little experimentation on my own. I found something interesting and useful.

Here's what I found. If I run this script in background, I do not experience any hangs when Time Machine wants to use that disk:

while :
do
for i in /Volumes/TM2
do
touch "$i/uppity"
ls -l  "$i/uppity"
done
sleep 300
done
 

Obviously all that is doing is keeping that particular disk from going to sleep. I used the "for i in /Volumes/TM2" loop so that I can add other disks if need be. With that running, I have no more regular interruptions of my other work.

That this works surprised me. I expected that I'd see a hang on initial use, but I do not. That says this is not a general flaw caused by not enough interrupts in the disk driver. It's a more specific flaw of not enough interrupts under some specific conditions. Apparently Time Machine causes those conditions.

Here are some of the links I found concerning OPPs.




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