On 2014-Mar-13 14:28, Hugo Mills wrote:
On Thu, Mar 13, 2014 at 08:12:44PM +0100, Michael Schuerig wrote:
My backup use case is different from the what has been recently
discussed in another thread. I'm trying to guard against hardware
failure and other causes of destruction.
I have a btrfs raid1 filesystem spread over two disks. I want to backup
this filesystem regularly and efficiently to an external disk (same
model as the ones in the raid) in such a way that
* when one disk in the raid fails, I can substitute the backup and
rebalancing from the surviving disk to the substitute only applies the
missing changes.
* when the entire raid fails, I can re-build a new one from the backup.
The filesystem is mounted at its root and has several nested subvolumes
and snapshots (in a .snapshots subdir on each subvol).
Is it possible to do what I'm looking for?
For point 2, yes. (Add new disk, balance -oconvert from single to
raid1).
For point 1, not really. It's a different filesystem, so it'll have
a different UUID. You *might* be able to get away with rsync of one of
the block devices in the array to the backup block device, but you'd
have to unmount the FS (or halt all writes to it) for the period of
the rsync to ensure a consistent image, and the rsync would have to
read all the data in the device being synced to work out what to send.
Probably not what you want.
Hugo.
I'm new; btrfs noob; completely unqualified to write intelligently on
this topic, nevertheless:
I understand your setup to be btrfs RAID1 with /dev/A /dev/B, and a
backup device someplace /dev/C
Could you, at the time you wanted to backup the filesystem:
1) in the filesystem, break RAID1: /dev/A /dev/B <-- remove /dev/B
2) reestablish RAID1 to the backup device: /dev/A /dev/C <-- added
3) balance to effect the backup (i.e. rebuilding the RAID1 onto /dev/C)
4) break/reconnect the original devices: remove /dev/C; re-add /dev/B to
the fs
I think this could be done online. Any one device [ABC] surviving is
sufficient to rebuild a RAID1 of the filesystem, or be accessed alone in
degraded fashion for disaster recovery purposes.
I think that would address point 1. Is my thinking horrible on this?
(again, noob to btrfs)
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