Something I noticed very quickly last night after migrating my Time Machine partition was the drive speed. If you have ever wondered why Time Machine sometimes takes forever on the “preparing backup” stage you’ll likely find this message in your console:

Node requires deep traversal:/ reason:kFSEDBEventFlagMustScanSubDirs|kFSEDBEventFlagReasonEventDBUntrustable|

In english, that means the backup cache (FSEvents) is out of sync (it’s the fancy background process that tracks drive activity and makes Time Machine so fast). “Deep Traversal” means that your entire drive must be scanned and compared to the Time Machine drive before the current backup can proceed. I regularly see this when rotating my backup drives, but not always.

On my old backup drives this takes a grand total of 9 minutes. After pointing Time Machine at the copy of it’s data on my new drive however, it took 33 minutes. The question is, should I care? Subsequent incremental backups went at their usual pace (4 seconds to backup 600kb and delete a backup from 24hrs ago).

This drive is going to sit (eventually) in a place that I don’t notice it except on Friday morning when I unplug it to be rotated. It’s not like my mac seems to notice the difference, USB drives don’t putting any noticeable load on my CPU. If it takes 1 minute or 1 hour it isn’t likely to bother me either way unless the mac goes to sleep in the middle and it doesn’t finish the backup.

Considering the benefits I get (no noise, drastically reduced power usage, only 1 cable instead of 3) I think I can live with this minor flaw.

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