Understanding btrfs and backups

Brian Wong wrote: a snapshot is different than a backup, with a snapshot you're still accessing a read-only version of the live filesystem. i don't know the specifics of btrfs but if you take daily snapshots, you should be able to restore a single file from the five-days-ago snapshot by browsing that snapshot's directory tree and then copying the file to the live version of the filesystem, if that makes sense.

in the snapshot case the live filesystem serves the same function as the full backup would if you did full backups then incrementals. the snapshots are the incrementals of the live filesystem, only going backwards in time whereas with backup you would take a full backup then go forward in time with incrementals. the filesystem takes care of making sure every snapshot is complete.

in the snapshot case redundancy is then more important because you may not have a bunch of full backups (i.e. full copies) lying around. so full backups still are useful.

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OK, I THINK I understand things a bit better. So from the point of view of restoring a single file, that functionality is there. Excellent. And I guess you're saying that because the snapshots are diffs off the live system, that I'd need a backup of the live system - ie snapshots wouldn't be enough. But what if my first snapshot was a clone of the system at that point (as it seems from the article) And I back that up to a separate drive. Let me illustrate with what I plan to do exactly.

Three hard drives: A, B, and C.

Hard drives A and B - btrfs RAID-1 so that if one drive dies I can keep using my system until the replacement for the raid arrives.

Hard drive C - gets (hourly/daily/weekly/or some combination of the above) snapshots from the RAID. (Starting with the initial state snapshot) Each timepoint another snapshot is copied to hard drive C.

So in the case of a file disappearing on me or being over-written or w/e - I reach into the directory of the snapshot that contains the file just as I would now with the backup.

So if that's what I'm doing, do snapshots become a way to do backups?

Thanks

An important distinction for anyone joining the conversation is that snapshots are *not* backups, in a similar way that you mentioned that RAID is not a backup. If a hard drive implodes, its snapshots go with it.

Snapshots can (and should) be used as part of a backup methodology - and your example is almost exactly the same as previous good backup examples. I think most of the time there's mention of an external "backup server" keeping the backups, which is the only major difference compared to the process you're looking at. Btrfs send/receive with snapshots can make the process far more efficient compared to rsync. Rsync doesn't have any record as to what information has changed so it has to compare all the data (causing heavy I/O). Btrfs keeps a record and can skip to the part of sending the data.

I do something similar to what you have described on my Archlinux desktop - however I haven't updated my (very old) backup script to take advantage of btrfs' send/receive functionality. I'm still using rsync. :-/ / and /home are on btrfs-raid1 on two smallish disks
/mnt/btrfs-backup is on btrfs single/dup on a single larger disk

See https://btrfs.wiki.kernel.org/index.php/Incremental_Backup for a basic incremental methodology using btrfs send/receive

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Brendan Hide
http://swiftspirit.co.za/
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