Mac – Time Machine: first backup keeps growing while backing up, never completes

backuptime-capsuletime-machine

I'm trying to use Time Machine for backups on an AirPort Time Capsule. The initial backup has been running for weeks (with some interruptions inbetween) but has never completed.

When I stop and restart the backup, it mounts the most recent APFS snapshot and copies files from it (as reported by lsof). The the total remaining size is, initially, rather small (~100MB-3GB). But whenever it's close to backing up that amount of data, the total size increases. It just keeps growing. There's no complete first backup and the sparsebundle on the Time Capsule is now almost twice the size of my local files.

The number of 'Used' bytes (as reported by df) of the snapshot filesystem that Time Machine is backing up from does not change, obviously.

Looking at the logs, I noticed something odd when I start the backup:

Warning: Bytes to copy estimate was negative (-107740)

How does macOS calculate this and why does it result in a negative amount?

This is the first log from /Volumes/Time Machine Backups/Backups.backupdb/XXXXX/$datetime.inProgress/.Backup.xxxxx.xxxx.log:

2018-10-11-15:43:12 - Starting backup

Previous snapshot:
  None

Initial backup of source: "Macintosh HD" (device: /dev/disk1s1 mount: '/' fsUUID: XBXBXBXB-XBXB-XBXB-XBXB-XBXBXBXBXBXB eventDBUUID: XCXCXCXC-XCXC-XCXC-XCXC-XCXCXCXCXCXC)
=== Starting backup loop #1 ===
  Will use FirstBackupCopier

Running preflight for "Macintosh HD" (device: /dev/disk1s1 mount: '/' fsUUID: XBXBXBXB-XBXB-XBXB-XBXB-XBXBXBXBXBXB eventDBUUID: XCXCXCXC-XCXC-XCXC-XCXC-XCXCXCXCXCXC)
  Excluding […]
  Excluding […]
  […]
  Should copy 3805858 items (308,93 GB) representing 75422739 blocks of size 4096. 259695348 blocks available.
Preflight complete for "Macintosh HD" (device: /dev/disk1s1 mount: '/' fsUUID: XBXBXBXB-XBXB-XBXB-XBXB-XBXBXBXBXBXB eventDBUUID: XCXCXCXC-XCXC-XCXC-XCXC-XCXCXCXCXCXC)
Time elapsed: 3.996 seconds

Processing preflight info
  Space needed for this backup: 330,41 GB (80665619 blocks of size 4096)
Finished processing preflight info

Copying items from "Macintosh HD" (device: /dev/disk1s1 mount: '/' fsUUID: XBXBXBXB-XBXB-XBXB-XBXB-XBXBXBXBXBXB eventDBUUID: XCXCXCXC-XCXC-XCXC-XCXC-XCXCXCXCXCXC)

All of the following logs look like this, with needed space decreasing until it's below 4GB:

2018-11-13-16:52:45 - Starting backup

Previous snapshot:
        None

Resuming from partial snapshot:
        /Volumes/Time Machine Backups/Backups.backupdb/XXXXX/2018-11-13-165241.inProgress/XAXAXAXA-XAXA-XAXA-XAXA-XAXAXAXAXAXA

Initial backup of source: "Macintosh HD" (device: /dev/disk1s1 mount: '/' fsUUID: XBXBXBXB-XBXB-XBXB-XBXB-XBXBXBXBXBXB eventDBUUID: XCXCXCXC-XCXC-XCXC-XCXC-XCXCXCXCXCXC)
=== Starting backup loop #1 ===
  Will use IncrementalBackupCopier

Processing preflight info
        Space needed for this backup: 2,25 GB (549204 blocks of size 4096)
        Preserving snapshot for resume /Volumes/Time Machine Backups/Backups.backupdb/XXXXX/2018-11-13-165241.inProgress/XAXAXAXA-XAXA-XAXA-XAXA-XAXAXAXAXAXA
Finished processing preflight info

Copying items from "Macintosh HD" (device: /dev/disk1s1 mount: '/' fsUUID: XBXBXBXB-XBXB-XBXB-XBXB-XBXBXBXBXBXB eventDBUUID: XCXCXCXC-XCXC-XCXC-XCXC-XCXCXCXCXCXC)

(Where XAXAXA… is different every time the backup resumes)

After I manually cancel the backup:

c :ERROR:-128, copy /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/XXXXX/2018-11-13-165240/Macintosh HD to /Volumes/Time Machine Backups/Backups.backupdb/XXXXX/2018-11-13-165241.inProgress/XDXDXDXD-XDXD-XDXD-XDXD-XDXDXDXDXDXD
Finished copying items for "Macintosh HD" (device: /dev/disk1s1 mount: '/' fsUUID: XBXBXBXB-XBXB-XBXB-XBXB-XBXBXBXBXBXB eventDBUUID: XCXCXCXC-XCXC-XCXC-XCXC-XCXCXCXCXCXC)
Time elapsed: 1 hour, 43 minutes, 35.000 seconds
        Copied 64889 items (17,25 GB)
        Copy stage canceled

Backup canceled.
Total time elapsed: 1 hour, 43 minutes, 44.000 seconds

As you can see, it originally wanted to backup 2,25 GB but copied more than 17 GB when I stopped it.

Edit: I've tried backing up to a directly connected hard drive and it worked flawlessly. The issue seems to only occur with TimeCapsule.

  • Other MacBooks on the same network, using the same Time Capsule do not have any issues
  • I have tried deleting and re-configuring the backup
  • I have tried deleting the Time Machine config
  • Disk Repair (fsck_apfs) reports no errors
  • There is sufficient space left on macOS and on the Time Capsule
  • The Time Capsule is connected via gigabit ethernet
  • My disk is encrypted by FileVault
  • The backup is encrypted
  • There are no excluded files in Time Machine settings
    • The first log reports a bunch of directories excluded by default
  • I'm on High Sierra. "Backup before updating" they said…

Best Answer

I might have another solution to your problem, as I have finally managed to resolve the issue with my Time Machine.

I have been using Dropbox with Smart Sync, which allows for files to appear on your computer hard drive, although not physically -- the files are actually in the Dropbox cloud. When creating a backup of my whole computer, the Dropbox Smart Sync files seemed to confuse Time Machine, since it wanted to back them up when they are not physically on the computer -- and thus the first backup kept growing and never completed.

In the Time Machine options, I tried excluding the files and folders using Smart Sync and the issue was resolved immediately.

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