On 2014/03/13 09:48 PM, Andrew Skretvedt wrote:
On 2014-Mar-13 14:28, Hugo Mills wrote:
On Thu, Mar 13, 2014 at 08:12:44PM +0100, Michael Schuerig wrote:
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.
For point 1, not really. It's a different filesystem
[snip]
Hugo.
I'm new
We all start somewhere. ;)
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
Its this step that won't work "as is" and, from an outsider's
perspective, it is not obvious why:
As Hugo mentioned, "It's a different filesystem". The two disks don't
have any "co-ordinating" record of data and don't have any record
indicating that the other disk even exists. The files they store might
even be identical - but there's a lot of missing information that would
be necessary to tell them they can work together. All this will do is
reformat /dev/C and then it will be rewritten again by the balance
operation in step 3) below.
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
Again, as with 2), /dev/A is now synchronised with (for all intents and
purposes) a new disk. If you want to re-add /dev/B, you're going to lose
any data on /dev/B (view this in the sense that, if you wiped the disk,
the end-result would be the same) and then you would be re-balancing new
data onto it from scratch.
Before removing /dev/B:
Disk A: abdeg__cf__
Disk B: abc_df_ge__ <- note that data is *not* necessarily stored in the
exact same position on both disks
Disk C: gbfc_d__a_e
All data is available on all disks. Disk C has no record indicating that
disks A and B exist.
Disk A and B have a record indicating that the other disk is part of the
same FS. These two disks have no record indicating disk C exists.
1. Remove /dev/B:
Disk A: abdeg__cf__
Disk C: gbfc_d__a_e
2. Add /dev/C to /dev/A as RAID1:
Disk A: abdeg__cf__
Disk C: _########## <- system reformats /dev/C and treats the old data
as garbage
3. Balance /dev/{A,C}:
Disk A: abdeg__cf__
Disk C: abcdefg____
Both disks now have a full record of where the data is supposed to be
and have a record indicating that the other disk is part of the FS.
Notice that, though Disk C has the exact same files as it did before
step 1, the on-disk filesystem looks very different.
4. Follow steps 1, 2, and 3 above - but with different disks - similar
end-result.
--
__________
Brendan Hide
http://swiftspirit.co.za/
http://www.webafrica.co.za/?AFF1E97
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