Can somebody explain what 'Overallocation' means in context of file system? I constantly see this message when trying to resize APFS partition, disk checks don't find any problems.


  • I have the same problem. From /var/log/fsck_apfs.log: ** Checking the snap/dev/disk0s2: shots. /dev/disk0s2: ** Verifying allocated space. /dev/disk0s2: warning: Overallocation Detected on Main device: (1116392+1) bitmap address (802394) /dev/disk0s2: ** The volume /dev/disk0s2 appears to be OK. /dev/disk0s2: fsck_apfs completed at Tue Jan 2 18:20:46 2018 /dev/disk0s2: – barksten Jan 2 '18 at 21:17

I booted in Recovery mode, open Terminal and run diskutil unmountDisk /dev/disk0 and after that fsck_apfs -y /dev/disk0 After this all was fine Maybe for you, the drive name is different, you can find it by running diskutil list


I would have commented on Peku2455's answer, but I don't have 50 rep. So here is the solution, based on Peku's answer (which is missing the -o switch to fix overallocation)

In Terminal in Recovery Mode (press Command+R while booting, select Utilities -> Terminal):

diskutil unmountDisk /dev/disk0
fsck_apfs -oy /dev/disk0

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