Time Machine backup cannot be reliably restored?

[DominikHoffmann]

I just saw this alert in Settings → General → Time Machine:

It reads:

Time Machine detected that your backups on "Backup" can not be reliably restored.
Time Machine must erase your existing backup history and start a new backup to correct this.

I am running macOS Sequoia and am backing up to a TrueNAS configured with a share configured for as a network backup destination for Time Machine.

How does this happen, and what exactly happened?

 

[Bigwaff]

How does this happen, and what exactly happened?

If you are using Time Machine to back up to a NAS, TM will create what is called a sparsebundle disk image on the NAS file system. The NAS file system is mounted on your Mac using the SMB protocol. Inside the sparsebundle disk image are the TM snapshots. Sometimes when a TM back up to NAS is interrupted, the TM data gets corrupted. You are after all writing data to a file system which resides within a mounted disk image, which itself is stored in an entirely different file system, which is accessed using a network file protocol which is layered over top. Many opportunities for things to go sideways.

Can you provide specifics about your TrueNAS system? What files system format are the disks? Which version of SMB?

 

[throAU]

Have more than one backup location.

Any backup program can and will fail, as can the media you're storing Time Machine on.

 

[DominikHoffmann]

If you are using Time Machine to back up to a NAS, TM will create what is called a sparsebundle disk image on the NAS file system. The NAS file system is mounted on your Mac using the SMB protocol. Inside the sparsebundle disk image are the TM snapshots. Sometimes when a TM back up to NAS is interrupted, the TM data gets corrupted. You are after all writing data to a file system which resides within a mounted disk image, which itself is stored in an entirely different file system, which is accessed using a network file protocol which is layered over top. Many opportunities for things to go sideways.

Can you provide specifics about your TrueNAS system? What files system format are the disks? Which version of SMB?

That’s a pretty good explanation and suffices for now. In that light a backup utility like UrBackup with a specific server component makes a lot of sense. If there are any interruptions, as there would be from maintenance reboots of the server or temporary removal of the laptop from the network, the server can find out exactly about where to continue when the connection gets re-established.