Why do I get different Available/Used spaces with Disk Info and df?

I have an early '13 MBP running Mavericks. Today, I got a SSD external drive today, and moved my virtual machines and Lightroom catalog files over to it to free up disk space, since 256GB goes fast.

However, when I run df -h in the terminal, I get the following:

Filesystem                          Size   Used  Avail Capacity  iused    ifree %iused  Mounted on
/dev/disk0s2                       233Gi  118Gi  115Gi    51% 31026495 30042945   51%   /
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:/19YSQKN76Bmz377sW5fOwp  233Gi  233Gi    0Bi   100%        0        0  100%   /Volumes/MobileBackups
/dev/disk1s2                       119Gi   36Gi   83Gi    31%  9537194 21637536   31%   /Volumes/Jason SSD

According to that, I have 118GB used and 115GB free. However, when I get info from the drive on the desktop, I get

That is a pretty startling discrepancy, and it exists after a reboot. What is the correct value and what is the reason why?

Solution 1:

Do you have Time Machine backups turned on? If so, the difference may be due to space used for "local snapshots" (essentially, backups to the local disk when your real backup disk is not available). Local snapshots are automatically deleted when the space is needed for something else (actually, it starts purging them when the volume reaches 80% full), so the Finder treats them as free space. du, on the other hand, sees them as used space (as do Disk Utility and System Information).

To find out if this is what's going on, take a look in System Information: From the Apple Menu, choose "About This Mac", then click "More Info", then select the Storage tab, and look for a light purple ("Backups") band on the usage chart. If it's around 47GB, this is the source of the difference.

If you want to disable this feature, you can use the Terminal command sudo tmutil disablelocal -- but I recommend leaving it enabled, as it does provide some useful protection against accidental deletion/file damage/etc when your real backup is not available.

EDIT: As @chillin pointed out, there's an additional source of confusion: df and the Finder are using different units to report the sizes. df is using Gibibytes (=1,073,741,824 bytes), while the Finder is using Gigabytes (=1,000,000,000 bytes). If the Finder had reported in GiB, it would've said Capacity: 232.96 GiB, Available: 166.41 GiB, Used: 66.55 GiB. So there was actually about 52 GiB used for the local snapshots.

Solution 2:

df is using Gi = Gibibytes, and "Get Info" is using GB = Gigabytes.