On 2016-03-22 16:42, Henk Slager wrote:
On Mon, Mar 21, 2016 at 4:43 AM, Chris Murphy <lists@xxxxxxxxxxxxxxxxx> wrote:
Hi folks,
So I just ran into this:
https://raid.wiki.kernel.org/index.php/Recovering_a_failed_software_RAID#Making_the_harddisks_read-only_using_an_overlay_file
This is a device mapper overlay file - not overlayfs.
For the repairs that are sometimes uncertain what's next, maybe this
is a viable option to avoid changing the file system? I'm thinking
chunk-recover might take up too much space, I'm not sure how that one
works, if chunks are just being read or if they have to be rewritten
or if it's just the chunk tree? But for 'btrfs check' and 'btrfs
rescue super-recover/zero-log' there should be very little being
written so the overlay idea might be a good step?
I used the info via this message:
http://permalink.gmane.org/gmane.comp.file-systems.btrfs/54178
to try to fix a 4x4TB disks RAID10 (some bad metadata, some nbytes 400 errors).
I used AoE (instead of NBD) to avoid that btrfs+kernel might get
confused by double UUID's.
I created 4x 10G sparse files for each bcached HDD. After the --repair
action had ended (apparently successful), du reported only 50M size on
disk for each of the sparse files. The fix operation lasted about 1.5
hours. After a mount and umount again of the 'just repaired fs', a
subsequent btrfs check still reported the same errors, although
reported in another sequence.
So the nbytes 400 errors actually did not get fixed ( while there were
also other errors; This in accordance to what Qu once noted, but at
that time older tools/kernel).
I actually do similar when I need to fix something other than my root
filesystem on my home server system. I run Xen though, so instead of
using AoE, I just unmount the filesystem, set up the snapshots, and then
attach them to the VM I use to build updates for the other VM's directly
via Xen's virtual block device protocol, and work with them from there.
Obviously not practical for most people, but it works well for my setup.
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