Mac – “About this Mac” not showing correct local backup size

backupdisk-spacetime-machine

"About this Mac" doesn't appear to be showing the correct information for backups. In fact, it doesn't have a category for backups, even though they are enabled. What I see is this:

2.25 GB free of 120.1 GB, Other: 78.58 GB

I tried using df -h, and here's what I got:

Filesystem                          Size   Used  Avail Capacity  iused  ifree %iused  Mounted on
/dev/disk1                         112Gi  110Gi  1.2Gi    99% 29009096 312630   99%   /
devfs                              187Ki  187Ki    0Bi   100%      646      0  100%   /dev
map -hosts                           0Bi    0Bi    0Bi   100%        0      0  100%   /net
map auto_home                        0Bi    0Bi    0Bi   100%        0      0  100%   /home
localhost:/ee6auZTwNjhqkHwrV5MmDo  112Gi  112Gi    0Bi   100%        0      0  100%   /Volumes/MobileBackups

The last line seems particularly confusing. So I tried something a little more pointed: I cd'd to /.MobileBackups, where I read that local backups are stored, and did this:

+~ $ sudo du -hc -d 0
 53G    .
 53G    total

If I'm reading this correctly, my backups are taking up 53 gigs of space, yet MacOS is not reporting this properly. What is wrong and how can I fix it? (Besides sudo tmutil disablelocal, which I might do anyway.)

EDIT: Okay, so now it gets weird. Following advice from here, I used sudo tmutil disablelocal, then restarted my computer (which took a while). Now /.MobileBackups is gone, but I get a similar report for disk space consumed in "About this Mac."

+~ $ df -h
Filesystem      Size   Used  Avail Capacity  iused   ifree %iused  Mounted on
/dev/disk1     112Gi  107Gi  4.4Gi    97% 28157320 1164406   96%   /
devfs          181Ki  181Ki    0Bi   100%      626       0  100%   /dev
map -hosts       0Bi    0Bi    0Bi   100%        0       0  100%   /net
map auto_home    0Bi    0Bi    0Bi   100%        0       0  100%   /home

And in the root directory, when I ran a command like this one:

+/ $ sudo du -hc -d 0; tput bel
104G    .
104G    total

Further searching reveals that my user directory is taking up 53 GB. I'm pretty confused now. One possible cause of this is that I changed my system clock by several months at one point, which seems to have messed with Time Machine.

Best Answer

TimeMachine uses hard links to do his magic for incremental backups. so if you do a du check on a TimeMachine backup volume, you won't see good results, because hardlinked files would count twice or more but in fact they use just one. if you check for disk usage, please exclude any form of hard links (backup directories). there is no option to "exclude hardlinks" similar to not follow symlinks, because the backup helper cannot distinguish between a hardlink and a real file.

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