Alan Hagge posted on Wed, 17 Sep 2014 10:13:01 -0700 as excerpted:
I know that sounds weird, but here's my scenario:
- Create a RAID1 filesystem (both data and metadata) using 2 same-sized
external USB drives - Copy data (backup of other filesystem) onto this
new filesystem - Dismount the filesystem - Split up the drives (keep one
at home, move one to offsite backup)
This way if I need to recover a file, I can mount the one drive I have
with -o ro,degraded to recover data. If there's a read error on the
backup drive during the copy, I can go to the offsite location, bring
back the 2nd drive and mount both and have RAID1 protection.
BUT...if I accidentally (because I forgot to use ro when mounting) or
purposely write data to the single drive in degraded mode, is it
possible to later mount both drives in RAID1 mode and resync them (as
opposed to having to do a replace operation on the out-of-sync drive,
which would force it to be completely rewritten)? If so, how would
btrfs know which drive is the master (ie. the updated one)?
Or is it not possible to write to a btrfs volume mounted in degraded
mode?
In general this works. However, as the thread linked in Piotr's reply
mentions, don't expect it to work for NOCOW files. (But you have to make
files nocow, so if you haven't, that shouldn't be an issue.)
Additionally, the newest version detection is based on file generation,
so you want to be SURE you don't separately mount first one device
writable and then the other, so they diverge not only from each other but
from the point of separation. If you split them, make SURE only the one
is mounted writable, so it'll be very clear which one has the updated
content. If you DO accidentally separately mount both of them writable,
I'd suggest using wipefs (or dd) to kill the btrfs magic on one of them
so there's no possibility of btrfs getting mixed up which is newer, and
then doing a btrfs replace to replace it with a new, current copy.
Meanwhile, again as noted in the Piotr's linked thread, the detection and
update isn't automatic. Once split with one mounted writable, when
rejoined you'll want to do a btrfs scrub to bring the other one current.
--
Duncan - List replies preferred. No HTML msgs.
Every nonfree program has a lord, a master --
and if you use the program, he is your master. Richard Stallman
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