Time Machine keeps validating backup, how to identify reason?

Question

I have a Time Machine backup of my Macbook running the latest MacOS (11.2.3) on my Time Capsule (so not a local drive).

Recently I've noticed that the "Validating backup"-step runs often, presumably daily, which is much more frequent than I expect. (I notice the drive spins up and keeps running). Time Machine reports backups are being done on a regular basis.

Is there a way to get more insight in how Time Machine is working on my machine, and why the validation step happens so frequently?

Answer 1

I was suspecting a broken file system on the Time Capsule, but needed proof. After yet another mysteriously cancelling verification step, I found https://superuser.com/q/1126990/7401 which ended up in

log show --last 3d --style syslog  --predicate 'senderImagePath contains[cd] "TimeMachine"' --info|less

which in turn let me find this log line:

2021-04-05 00:48:38.359222+0200  localhost backupd[276]: (TimeMachine) [com.apple.TimeMachine:General] fsck returned an unknown termination status: 8

So, the file system check program reported an error message but Time Machine didn't let me know. Apparently 8 indicates unrepairable error, which is a rather bad sign. I then mounted the "Data" drive holding the backupbundle file on Time Capsule manually, and as Disk Utility did not want to mount the backupbundle, I found https://apple.stackexchange.com/a/39842/3157 resulting in

sudo hdiutil attach -nomount -readwrite /Volumes/Data/XXX.backupbundle

giving

/dev/disk2              GUID_partition_scheme           
/dev/disk2s1            EFI                             
/dev/disk2s2            Apple_HFS            

Now the Time Machine backup shows in Disk Utility, and I could run First Aid on the drive which slowly but successfully repaired the filesystem.