Re: how stable are snapshots at the block level?

[Chris Mason <chris.mason@oracle.com>]

On Sun, Oct 23, 2011 at 09:45:10AM +0200, Mathijs Kwik wrote:
> Hi all,
> 
> I'm currently doing backups by doing a btrfs snapshot, then rsync the
> snapshot to my backup location.
> As I have a lot of small files and quite some changes between
> snapshots, this process is taking more and more time.
> I looked at "btrfs find-new", which is promissing, but I need
> something to track deletes and modifications too.
> Also, while this will help the initial comparison phase, most time is
> still spent on the syncing itself, as a lot of overhead is caused by
> the tiny files.
> 
> After finding some discussion about it here:
> http://www.backupcentral.com/phpBB2/two-way-mirrors-of-external-mailing-lists-3/backuppc-21/using-rsync-for-blockdevice-level-synchronisation-of-backupp-100438
> 
> I found that the official rsync-patches tarball includes the patch
> that allows syncing full block devices.
> After the initial backup, I found that this indeed speeds up my backups a lot.
> Ofcourse this is meant for syncing unmounted filesystems (or other
> things that are "stable" at the block level, like LVM snapshot
> volumes).
> 
> I tested backing up a live btrfs filesystem by making a btrfs
> snapshot, and this (very simple, non-thorough) turned out to work ok.
> My root subvolume contains the "current" subvolume (which I mount) and
> several backup subvolumes.
> Ofcourse I understand that the "current" subvolume on the backup
> destination is broken/inconsistent, as I change it during the rsync
> run. But when I mounted the backup disk and compared the subvolumes
> using normal file-by-file rsync, they were identical.
> 
> Can someone with knowledge about the on-disk structure please
> confirm/reject that subvolumes (created before starting rsync on the
> block device) should be safe and never move by themselves? Or was I
> just lucky?
> Are there any things that might break the backup when performed during rsync?
> Like creating/deleting other subvolumes, probably defrag isn't a good
> idea either :)

The short answer is that you were lucky ;)

The big risk is the extent allocation tree is changing, and the tree of
tree roots is changing and so the result of the rsync isn't going to be
a fully consistent filesystem.

With that said, as long as you can mount it the actual files in the
snapshot are going to be valid.  The only exceptions are if you've run a
filesystem balance or removed a drive during the rsync.

-chris