My backups seem to go well backing up to my NAS however randomly after a few weeks I get the following error. Several users get this error intermittently however I have yet to find a solution. Any ideas?
Time Machine completed a verification of your backups on “NAS”. To
improve reliability, Time Machine must create a new backup for you.Click Start New Backup to create a new backup. This will remove your
existing backup history. This could take several hours.Click Back Up Later to be reminded tomorrow. Time Machine won’t
perform backups during this time.
Best Answer
Suitability of HFS Plus
Whilst Time Machine must use HFS Plus for most things, it's worth noting that the file system is not ideally suited for the task.
An example
Coincidence: a few hours after my first edition of this answer, my own Time Machine Backups volume (a sparse bundle disk image) suffered a file system failure. I'm certain that the underlying storage is OK – a ZFS pool scrubbed, without error, before and after the HFS Plus failure. For the record:
the message at 2013-06-07 22:28:49 is eye-catching, but expected in my case (symptom of a bug involving HFS Plus; a corruption that involves AppleFSCompression) – probably negligible in the context of this answer
the message at 2013-06-07 22:28:51 may be more relevant to the file system failure.
/private/var/log/fsck_hfs.logthen showed:Confirming that no error affected the underlying storage at the time:
In simple terms
Whilst we'd like a solution, file system failures such as this seem to be:
In the absence of a good solution, my best advice is to not rely on a single Time Machine backup. The risk of eventual failure and inability to repair is simply too high.
Degrees of failure
In the past I sometimes forced
fsck_hfs(8) to rebuild b-tree files … with limited success but not certainty. Whilst a file system may appear to be OK (in Disk Utility and the like) I would not no longer trust it for Time Machine backup or restoration purposes.In the most recent case (above) multiple applications of force (multiple rebuilds of the catalog b-tree, a rebuild of the extended attributes b-tree and a rebuild of the extents b-tree) have not led to a verifiable file system. I have debug logs from these attempts, which I'll not begin to summarise here; they're massive.
With locally attached disks (USB 2.0), attempts to repair Time Machine backup volumes can be extraordinarily time consuming. Wireless – over AFP – you may find the time required intolerable.
Time Machine - Troubleshooting – C13. "…Time Machine must create a new backup for you." (James Pond) includes much useful information. Essentially:
When OS X reports that an HFS Plus file system appears to be OK, there may be significant issues with the disk – issues that OS X simply can not detect.
As corruption has occurred more than once, there may be a problem with:
What make and model is the NAS?
Hard disks of the NAS
If the OS of the NAS allows you to verify integrity of blocks on its disks: please do so.
If the OS of the NAS lacks that capability, then aim to boot the hardware with a different OS that's more suitable for testing. Options might include Ubuntu and a run of badblocks.
Checks of this type: