Time Machine Restore: Why are my data folders larger than the Time Machine backup? Does a TM restore verify data?

[Avery1]

Any idea why my data copied from a Time Machine backup is larger than the data on the backup disk? Does Time Machine use some compression or de-duplication? Is there another way to verify all data is present and not-corrupted (maybe TM does this when it performs a restore?)


Background:
Had an iMac die, bought a new one, and started with a clean install of Ventura + copying over my data folders.

Left is the backup. Right is the new computer folder. I did not ask Time Machine to do a restore, per se - I just copied and pasted the files from the drive. Some folders are identical size, including folders where I know the data is already compressed (e.g. music, vmware VM). In all cases, the number of files matches.

Note: Timemachine backup was from Catalina

 

[BrianBaughn]

I don't know. Why don't you run a "diff" command in Terminal to compare?

How to use the macOS command line to compare two folders’ contents in Terminal

Use a built-in Unix program to quickly compare two folders' contents.

www.macworld.com

 

[Avery1]

Agreed that would be useful, though on an 86GB folder on an external drive that might be a long run.

 

[Grumpus]

What is the output of the following commands?

diskutil info / | grep -i block                 # the device containing your root fs
diskutil info /Volumes/whatever | grep -i block # the device containing your backups

 

[Avery1]

What is the output of the following commands?

diskutil info / | grep -i block                 # the device containing your root fs
diskutil info /Volumes/whatever | grep -i block # the device containing your backups

Here you go.

/> /volumes % diskutil info / | grep -i block

Device Block Size: 4096 Bytes

Allocation Block Size: 4096 Bytes

/> % diskutil info /volumes/AB2 | grep -i block

Device Block Size: 512 Bytes

Allocation Block Size: 8192 Bytes


Now, that said, I did talk with Apple. They indicated there is compression used with TimeMachine backups, and they did not think that when using TimeMachine to restore a folder that it performs any corruption checks or data validation.

 

[Grumpus]

Apologies, I was in too much of a hurry and noticing only the on-disk sizes, which can vary a lot between devices with different allocation block sizes, especially if you have a lot of small files. Is there some reason not to use migration assistant to restore the data?

 

[NoBoMac]

^^^This. As mentioned, allocation block sizes, type of files. Also, what seems to be in this case, the TM drive is HFS+ and new Mac is APFS, so dealing with different data structures/tables to manage data on disk (ie. volume/system overhead to manage files is different).

 

[Avery1]

Ok, thanks all. That helps clarify that it is expected.

Migration assistant... For a lot of different reasons I wanted a clean install. Just need my files, not every configuration made in the last 8 years. Sometimes just best to start clean and get out all the gremlins, taking only what you want.

 

[Grumpus]

Migration assistant... For a lot of different reasons I wanted a clean install. Just need my files, not every configuration made in the last 8 years. Sometimes just best to start clean and get out all the gremlins, taking only what you want.

With MA you don't have to install everything on the backup, you can pick and choose. See #7 here.

 

[Avery1]

Call me old school, but I like moving the files myself... takes away all questions about what was moved or done behind the scenes. Not too hard.

 

[Grumpus]

... takes away all questions about what was moved or done behind the scenes.

Obviously not, but suit yourself.