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Comment by pfranz | Hacker News Reader

I have a nightly snapshot and do the same if there's an issue. To be fair, I've only ever hit these "verification" errors when I manually mounted and mucked around with the backup and a backup started at the same time--all over the network.

I've never had this issue when using an external hard drive connected via USB and I've never had it happen when I wasn't mucking with it. For awhile I was able to "fix" these errors by running diskutil and resetting the state (my Time Machine server was a flaky Raspberry Pi at the time).

All in all, it is concerning when you "lose" your backup no matter the circumstances.

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In my case, the NAS is a Synology server. I don't know how to determine authoritatively where the fault lies, but I trust it more than I trust Time Machine. (Although I am using btrfs on that volume so maybe it's a wash.) I'm sure I've disconnected my Mac from my Thunderbolt dock (and thus the active NIC) and/or put my Mac to sleep mid-backup many times, though. This is something that I would like it to be robust to but may be "the problem".

Interesting idea to reset the state. I think I can take snapshots with Synology. I'm unsure how to manage the details though:

* I let Time Machine starts backups automatically whenever it pleases. Does it have a hook to run a command to take a snapshot?

* Does it do the verification every time, or (say) 1 in 10? If the latter, going back to the most recent snapshot isn't enough. And if you do say 3 backups then get rid of #3, can it take incremental #4 on top of #2, or will it complain that the #3 state it expects is absent?

I really should just set up a different backup system. I do at least have Google Drive sync on most everything important. It's not a "real" backup as another poster pointed out, but it's better than nothing.

> I let Time Machine starts backups automatically whenever it pleases. Does it have a hook to run a command to take a snapshot?

I don't know, but you can trigger backups on demand using tmutil(8), and that's what I do.

> And if you do say 3 backups then get rid of #3, can it take incremental #4 on top of #2, or will it complain that the #3 state it expects is absent?

You can delete arbitrary backup(s). "Classic" Time Machine backups are snapshots deduped using hard links. HFS+ even supports directory hard links.

> In my case, the NAS is a Synology server.

Same here. I'm also using BTRFS (I've not had a problem and my understanding is Synology doesn't use the flakier features of BTRFS). I can't remember if I'm using AFP or SMB (SMB I think is the only one Apple supports), but my backup is encrypted and I have a specific user. I'm not sure BTRFS matters all that much because the backup is a disk image. When I've had them get corrupted I've run disk utility on the disk image to fix things--there's a lot of layers to unwind.

I hear stories of people having issues with similar setups, but I haven't (I know that's a sucky answer to hear). Years and years ago I felt like I would get a corrupt backup when I closed my lid during a network backup (I believe this was pre-APFS), but I haven't noticed an issue in years. I have it auto-backup and it usually does it at night when the lid is closed and it's charging.

In Snapshot Replication I have a daily snapshot created at midnight with a 2 snapshot retention. I tend not to run out of space, so I'm fine with any deletes taking a few days to roll off. "Recover" under "Recovery" can restore old snapshots easily. I do this for most all of my Shared Folders.

> * Does it do the verification every time, or 1 in 10? If the latter, going back to the most recent snapshot isn't enough. And if you do say 3 backups then get rid of #3, can it take incremental #4 on top of #2, or will it complain that the #3 state it expects is absent?

I don't believe it does the verification every time. I can't remember what the criteria is (I assumed it was similar to fsck on boot for Linux). You can run verify manually (Option-click the menu bar) and I'm sure there's a command line--so maybe you want to schedule your backups explicitly and verify every time? I've always had a corrupt backup when I've manually mounted the disk image and mucked around, so it's easy to restore within two days. I thought I might have issues with the client/server state mismatch, but haven't in practice (I've only needed to two this a few times). I haven't kept close track to see if I lose any in between backups--but I restore so rarely I wouldn't miss them.

> I really should just set up a different backup system. I do at least have Google Drive sync on most everything important. It's not a "real" backup as another poster pointed out, but it's better than nothing.

I've kept to the 3-2-1 backup rule and kept bootable backups local and non-bootable ones remote.

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