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TOPIC: Can it really take this long?

Can it really take this long? 09 Jul 2019 19:58 #11237

My Time Machine backup drive is a 1.6TB volume on a 2TB external USB 3.0 drive. It has about 1TB of backup data on it.

I thought I'd try doing a Volume Rebuild with TTP 11.0.3. Currently I'm over 13 hours into it and it's still flashing "Rebuilding Directory Structures".

Is it possible that this is normal? I imagine that Time Machine volumes probably have a complex directory structure due to all the hard links, but this seems crazy. Disk Utility did its "First Aid" checking in just a few minutes.

The machine is a decently-fast 2015 quad core MacBook Pro with 16GB RAM, running OS 10.14.5.

Related question: can TTP keep the machine from sleeping and pausing its work? I set Energy Saver preferences to "Prevent computer from sleeping automatically when the display is off" just to be safe, but I'd like to know if this is necessary.
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Can it really take this long? 09 Jul 2019 21:31 #11238

TechTool Pro should be able to keep the computer from going to sleep while it is performing a task.

We do not recommend that you try to rebuild the disk directory on a Time Machine volume. When the disk directory is rebuilt, the two main disk directory files, the Catalog B-Tree and the Extents B-Tree, must both be able to be held in RAM simultaneously. Roughly speaking, if you have 40 backups on your Time Machine volume, its disk directory is 40 times the size of the disk directory of a volume that could hold a typical example of one of those 40 backups. Nobody has that much RAM.

We recommend that you exclude the Time Machine volume from the File Structures test, because the files have already been tested on their original source volumes.

I strongly recommend running the Volume Structures test and the Surface Scan test to make sure that the complex directory of the Time Machine volume has not become corrupted, and to make sure that the drive is not developing unremapped bad blocks.

Finally, Time Machine should not be the only type of backup you have. Please see the impolitely-titled, slightly dated, but still very helpful:

11 Stupid Backup Strategies
MicroMat Inc
Makers of TechTool
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Can it really take this long? 09 Jul 2019 21:58 #11239

Thanks. I kind of suspected that might be the case. I just came back to the computer and TTP had posted this message:
Rebuild Error Encountered — Unable to Rebuild
Is this the expected result from running out of RAM? (If so, why not just say "Can't rebuild—not enough RAM"?)
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Can it really take this long? 09 Jul 2019 22:04 #11240

You are welcome.

I do not know all of the possible causes for the volume rebuild error. If you want to explore the issue, please send a message to This email address is being protected from spambots. You need JavaScript enabled to view it. . There may be some error messages in a log file that will provide more information.

Be sure to run the Surface Scan to check the drive containing the Time Machine volume for unremapped bad blocks. Also, try using Disk Utility to verify the volume, or the Volume Structures test in TechTool Pro. Both of these will take a long time on a Time Machine volume. The amount of data is not the issue, but the number of files, and how many backups are on the Time Machine volume. There are files that are actually written to the Time Machine only once, but if there are 40 backups, the one file has 40 different disk directory entries, each of which must be checked.
MicroMat Inc
Makers of TechTool
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Can it really take this long? 09 Jul 2019 22:05 #11241

It looks like your "Volume Structures" test is pretty much the same thing as Disk Utility's "First Aid". Right?

Not to get too deep into the weeds here, but would it be fair to say that Volume Rebuild rebalances the b-tree whereas Volume Structures just validates it?
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Can it really take this long? 09 Jul 2019 22:09 #11242

The Volume Structures test is a read-only test that checks the disk directory for errors . If you run Disk Utility to verify a disk without attempting to repair it, you have something conceptually similar.

The Volume Rebuild feature builds a new Catalog B-Tree and Extents B-Tree file in RAM, then writes it to the volume. Running Disk Utility with the option to fix any errors encountered is something conceptually similar, although Disk Utility does not re-order the disk directory entires in a way that makes the Finder slightly more efficient.
MicroMat Inc
Makers of TechTool
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Can it really take this long? 09 Jul 2019 22:15 #11243

Right. I remember back when I used to use Disk Warrior, they always said that other utilities just "patch" the b-tree, while they regenerate it. I guess same for TTP.

(Just FYI, I'm not having any problems with the drive, just wanted to verify everything was OK. I wonder when Apple will support APFS volumes for Time Machine. Should make things a lot more efficient, I think.)
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Can it really take this long? 09 Jul 2019 22:20 #11244

Whether the disk directory is "patched" or completely rebuilt is a choice the programmer makes. As long as the resulting directory is free of errors, the approach is not crucial. The advantage of rebuilding the directory is that it allows, in the case of HFS+ volumes, the opportunity to re-arrange the directory entries in a fashion that Micromat and Alsoft agree makes the Finder slightly more efficient.

Apple has still not provided detailed documentation for writing to APFS volumes.

There is an important discussion at MacInTouch about APFS clones:

www.macintouch.com/community/index.php?t...89/page-6#post-18665
MicroMat Inc
Makers of TechTool
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