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I have an external 3TB FW800 drive set up to be my Time Machine drive. I noticed some unusually high drive activity, and opened up Time Machine to see if it was backing up. Nope, TM showed previous backup was done about 30-40 minutes prior, and there was no activity in the app.

So to be overly cautious, I popped open Disk Utility, selected the external drive, and started a "Verify Disk." I've done this hundreds of times before on this and other drives...not talking about deep, hardcore tech support here. Simple stuff.

Well, for some reason, it's taking a VERY long time. It's been moving along, progressing through the various steps, but after 30 minutes, it still estimates another 1hr 35mins to go. It has gotten to "Checking multi-linked directories" and been sitting there for about 10-15 mins.

The drive has worked flawlessly for over a year, and is about 85% full (hasn't started deleting old backups though), but I don't ever remember it taking THIS long to verify.

Is there something about TM volumes that makes Verify Disk take longer, or should I be concerned about the hardware? What could be the reason for drive activity if TM wasn't backing up?

bmike
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johnneese
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1 Answers1

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The storage mechanism where each backup is a full backup is what takes so incredibly long to verify a Time Machine drive. The multi-linked directories is precisely why Time Machine makes the check so slow and precisely why each backup doesn't save two copies of the same file.

You can benchmark this by timing your Mac and see how long it takes to verify the source drive. That process is limited by the IO rate (or latency) of the storage subsystem. It doesn't read the contents of each file (which would measure the bandwidth of the storage subsystem).

The number of input-output operations per seconds (iops) can be graphed in Activity Monitor if you select that option.

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Each backup interval doubles the number of tops required by Disk Utility. Suppose your Time Machine backup drive contains two months of backups and your Mac doesn't sleep:

  • 24 backups - one per hour
  • 31 backups - one per day (the rest are deleted/pruned)
  • 4 backups - one per week (the rest are pruned)

I could be off by one or two, but you're in the neighborhood of 60 copies of each file on your Mac that gets backed up (not all of them do - so maybe the discount is 15% of files don't back up). So if Disk Utility takes 15 minutes to check your Mac - the Time Machine drive will take 15 hours to check. That assumes the external drive is rated for the same iops as the internal drive - that typically isn't the case. You might estimate external drives are a factor of 2 slower due to iops being slower.

Now you're 30 hours waiting (or 25 hours if you discount 15% of the files).

In practice, I can't check my Time Machine volumes since I have three macs backing up three months (or far longer) to one drive. I simply can't wait for that drive to get "verified" or "fixed". Instead I buy a second drive and rotate them weekly or monthly.

bmike
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    Thanks for the great answer. I was already considering a redundant TM drive for data protection...you just helped me over the fence! – johnneese Sep 23 '15 at 16:52
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    Well said @johnneese - Multiple backup destinations is my favorite feature added to Time Machine lately. If you have a business requirement or your time is precious - adding yet a third drive to make a SuperDuper/DiskUtility restore/CarbonCopyCloner bootable backup is my trifecta of backup bliss. – bmike Sep 23 '15 at 16:58
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    After a close call last year, I became quite diligent with backup practices. I'm using SuperDuper to make multiple (weekly) copies of my data drives, and have another bootable external drive with monthly images of my system drive (in case TM fails). Also have a base OS X Mountain Lion image, just in case I want a fresh build. :) – johnneese Sep 23 '15 at 17:06
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    Yes - carrying around USB drives with InstallESD and using munki scripts is the natural progression of this digital angst sickness disorder :-) – bmike Sep 23 '15 at 17:12
  • Uh oh, you're saying it can get worse? I thought it was bad enough already. Thanks for the support (both tech and psych). And BTW, it's not paranoia if the files really are trying to become lost. – johnneese Sep 23 '15 at 17:18
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    Good answer. "Checking multi-linked directories" took about 5 hours on my 460 GB Time Machine partition with 76 backups. Tip: If you want to see the progress, in Activity Monitor, at the bottom of the "Disk" tab, as in bmike's screenshot, look at the "Data read:" number. Assuming that you just launched Disk Utility to do this, it will finish when that number gets up to roughly the used size of your Time Machine. – Jerry Krinock Mar 13 '16 at 20:00
  • Why won't a First Aid check quit, even if it's not doing any repairs? Seems to require a force-quit. – Marnix A. van Ammers Jan 15 '19 at 15:37
  • @MarnixA.vanAmmers that is the situation I'm in; my running is assumption is that it IS doing checking/repairs—just taking potentially many, many hours due to the Time Machine multilink aspect described in here. – Ezekiel Victor Mar 14 '19 at 22:56