It’s fairly obvious why stopping a service while backing it up makes sense. Imagine backing up Immich while it’s running. You start the backup, db is backed up, now image assets are being copied. That could take an hour. While the assets are being backed up, a new image is uploaded. The live database knows about it but the one you’ve backed up doesn’t. Then your backup process reaches the new image asset and it copies it. If you restore this backup, Immich will contain an asset that isn’t known by the database. In order to avoid scenarios like this, you’d stop Immich while the backup is running.

Now consider a system that can do instant snapshots like ZFS or LVM. Immich is running, you stop it, take a snapshot, then restart it. Then you backup Immich from the snapshot while Immich is running. This should reduce the downtime needed to the time it takes to do the snapshot. The state of Immich data in the snapshot should be equivalent to backing up a stopped Immich instance.

Now consider a case like above without stopping Immich while taking the snapshot. In theory the data you’re backing up should represent the complete state of Immich at a point in time eliminating the possibility of divergent data between databases and assets. It would however represent the state of a live Immich instance. E.g. lock files, etc. Wouldn’t restoring from such a backup be equivalent to kill -9 or pulling the cable and restarting the service? If a service can recover from a cable pull, is it reasonable to consider it should recover from restoring from a snapshot taken while live? If so, is there much point to stopping services during snapshots?

  • Avid Amoeba@lemmy.caOPEnglish
    1·
    5 months ago

    Contents of application memory lost, yes. Contents of unflushed OS buffers, no. Your db will be fsyncing (or moral equivalent thereof) if it’s worth the name.

    Good point. I guess kill -9 is somewhat less catastrophic than a power-yank. If a service is written well enough to handle the latter it should be able to handle the former. Should, subject to very interesting bugs that can hide in the difference.

    This is an aside; backing up from a volume snapshot is half a reasonable idea. (The other half is ensuring that you can restore from the backup, regularly, automatically, and the third half is ensuring that your automated validation can be relied on.)

    I’m currently thinking of setting up automatic restore of these backups on the off-site backup machine. That is the backups are transferred to the off-site machine, restored to the dirs of the services, then the services are started. This should cover the second half I think. Of course those services can’t be used to store new data because they’ll be regularly overwritten with every backup. In the event of a hard snafu where the main machine disappears, I could stop the auto restore on the off-site machine and start using the services from it, effectively making it the main machine. If this turns out to be reasonable and working, I might trash all of the file-based backup-and-transfer mechanisms and switch to ZFS send/recv. That should allow to shrink the data delta between main and off-site to minutes instead of hours or days. Does this make any sense?