On Wed, Sep 29, 2021 at 5:48 PM Wols Lists <antli...@youngman.org.uk> wrote: > > An LVM snapshot creates a "copy on write" image. I'm just beginning to > dig into it myself, but I agree it's a bit confusing.
So, snapshots in general are a solution for making backups atomic. That is, they allow a backup to look as if the entire backup was taken in an instant. The simplest way to accomplish that is via offline backups. Unmount the drive, mount it read-only, then perform a backup. That guarantees that nothing changes between the time the backup starts/stops. Of course, it also can mean that for many hours you can't really use the drive. Snapshots let you cheat. They create two views of the drive - one that can be used normally, and one which is a moment-in-time snapshot of what the drive looked like. You backup the snapshot, and you can use the regular drive. With something like LVM you probably want to unmount the filesystem before snapshotting it. Otherwise it is a bit like just pulling the plug on the PC before doing an offline backup - the snapshot will be in a dirty state with things in various state of modification. This is because LVM knows nothing about your filesystem - when you run the snapshot it just captures the state of every block on the disk in that instant, which means dirty caches/etc (the filesystem caches or file buffers wouldn't be covered, because those are above the LVM layer, but caches below LVM would be covered, like the physical disk cache). Some filesystems like btrfs and zfs also support snapshotting. Those are much safer to snapshot while online, since the filesystem is aware of everything going on. Of course even this will miss file buffers. The cleanest way to snapshot anything is to ensure the filesystem is quiescent. This need only be for a few seconds while snapshotting though, and then you can go right back to using it. If you do a full backup on a snapshot, then if you restore that backup you'll get the contents of the filesystem at the moment the snapshot was taken. You can also use snapshots as a sort-of backup as well. Of course those are on the same physical disk so it only protects you against some types of failures. Often there is a way to serialize a snapshot, perhaps incrementally - I know zfs and btrfs both allow this. This can be a super-efficient way to create backup files since the filesystem metadata can be used to determine exactly what blocks to back up with perfect reliability. The downside of this is that you can only restore to the same filesystem - it isn't a simple file-based solution like with tar/etc. So as to not do a second reply I want to address an earlier question, regarding backups that span multiple volumes. This is a need I have, and I haven't found a simple tool that does it well. I am VERY interested in suggestions here. Tar I think can support something like this, since it is common for tape backups to span multiple volumes. I have no idea how clean that is. Most tools want you to use LVM or a union filesystem or whatever to combine all the drives into a single mountpoint and let linux manage the spanning. I'm using bacula which can sort-of handle multiple volumes, but not very cleanly, and it is an overly complex tool for backing up one system. I wouldn't really endorse it wholeheartedly, but it can be made to work. -- Rich
