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I've lost my backup twice. OS says I have to redo the full backup because there's something wrong with it. I have a Time Capsule.

Pamar 7y ago
If I understand this correctly it corrupted an already existing on-going Time Machine backup so that you had to start just as if you had bought the Time Capsule? And this was made explicit by an OS error at the end (or at the start) of a backup operation?
- mindajar 7y ago
  If TM detects an I/O error while writing to a networked backup volume, it aborts the backup and forces a filesystem check on the remote disk image at the start of the next backup. This fails, because the filesystem actually is corrupt and has been for some time. You had no idea, of course, because TM only reports four-horsemen level errors in the UI, and silently eats the rest. You even had a good run of "successful" backups afterwards, so long as you managed to avoid changing any files in directories with damaged metadata in your backup. Anyway, as your digital life flashes before your eyes, TM helpfully offers to delete all your backups and start with a fresh disk image now, or delete all your backups and start with a fresh disk image later. And you can't blame it, really, because the filesystem has had some unknown, nonzero amount of corruption for an unknown, nonzero amount of time. For all anyone knows or can prove, the whole backup is random garbage. This fractal of failure is specific to backups over WiFi. Backups to USB-connected disks aren't that reliable either, because HFS+, but USB backups have all the nines of reliability compared to network backups.
nonninz 7y ago
That wasn't introduced by Catalina though, it has been there since forever. For what I understood it happens when the macbook is forced to go to sleep/loses connectivity while in the middle of a backup. It doesn't cleanly unmount the remote file system and it causes data corruption. There are some ways of "restoring" the remote FS but they may or may not work, are very slow, opaque (what actually was fixed?) and flaky. I just switched to an external USB drive for Time Machine and not looking back

Pamar 7y ago

If I understand this correctly it corrupted an already existing on-going Time Machine backup so that you had to start just as if you had bought the Time Capsule? And this was made explicit by an OS error at the end (or at the start) of a backup operation?

mindajar 7y ago
If TM detects an I/O error while writing to a networked backup volume, it aborts the backup and forces a filesystem check on the remote disk image at the start of the next backup. This fails, because the filesystem actually is corrupt and has been for some time. You had no idea, of course, because TM only reports four-horsemen level errors in the UI, and silently eats the rest. You even had a good run of "successful" backups afterwards, so long as you managed to avoid changing any files in directories with damaged metadata in your backup. Anyway, as your digital life flashes before your eyes, TM helpfully offers to delete all your backups and start with a fresh disk image now, or delete all your backups and start with a fresh disk image later. And you can't blame it, really, because the filesystem has had some unknown, nonzero amount of corruption for an unknown, nonzero amount of time. For all anyone knows or can prove, the whole backup is random garbage. This fractal of failure is specific to backups over WiFi. Backups to USB-connected disks aren't that reliable either, because HFS+, but USB backups have all the nines of reliability compared to network backups.

nonninz 7y ago

That wasn't introduced by Catalina though, it has been there since forever. For what I understood it happens when the macbook is forced to go to sleep/loses connectivity while in the middle of a backup. It doesn't cleanly unmount the remote file system and it causes data corruption. There are some ways of "restoring" the remote FS but they may or may not work, are very slow, opaque (what actually was fixed?) and flaky. I just switched to an external USB drive for Time Machine and not looking back