Rich Freeman <ri...@gentoo.org> writes: > On Tue, Jan 19, 2016 at 5:22 PM, lee <l...@yagibdah.de> wrote: >> "J. Roeleveld" <jo...@antarean.org> writes: >> >> How does that work? IIUC, when you created a snapshot, any changes you >> make to the snapshotted (or how that is called) file system are being >> referenced by the snapshot which you can either destroy or abandon. >> When you destroy it, the changes you made are being applied to the >> file system you snapshotted (because someone decided to use a very >> misleading terminology), and when you abandon it, the changes are thrown >> away and you end up with the file system as it was before the snapshot >> was created. >> >> In any case, you do not get multiple versions (which only reference the >> changes made) of the file system you snapshotted but only one current >> version. >> >> Do you need to use a special file system or something which provides >> this kind of multiple copies when you make snapshots? >> > > And that is exactly what zfs and btrfs provide. Snapshots are full > citizens. If I create a snapshot of a directory in btrfs it is > essentially indistinguishable from running cp -a on the directory, > except the snapshot takes only seconds to create almost entirely > regardless of size, and takes almost no space until changes are made. > Later I can delete the snapshot, or delete the original, or keep both > indefinitely making changes to either.
Hm, I must be misunderstanding snapshots entirely. What happens when you remove a snapshot after you modified the "original" /and/ the snapshot? You destroy at least one of them, so you can never get rid of the snapshot in a non-destructive way? My understanding is that when you make a snapshot, you get a copy that doesn't change which you can somehow use to make backups. When the backup is finished, you can remove the snapshot, and the changes that were made in the meantime are not lost --- unless you decide to throw them away when removing the snapshot, in which case you get a rollback. To make things more complicated, I've seen zfs refusing to remove a snapshot and saying that something is recursive (IIRC), and it didn't make any sense anymore. So I left everything as it was because I didn't want to loose data, and a while later, I removed this very same snapshot without getting issues as before. Weird behaviour makes snapshots rather scary, so I avoid them now. There seems to be some sort of relationship between a snapshot and the "original" which limits what you can do with a snapshot, like the snapshot is somehow attached to the "original". At least that makes some sense to me because no real copy is created when you make a snapshot. But how do you detach a snapshot from the "original" so that you could savely modify both?
