Peter Foley posted on Thu, 06 Aug 2015 15:17:04 -0700 as excerpted:
I have an btrfs volume that spans multiple disks (no raid, just single),
and earlier this morning I hit some hardware problems with one of the
disks.
I tried btrfs dev del /dev/sda1 /, but btrfs was unable to migrate the
1gb that appears to be causing the read errors.
See http://sprunge.us/aeZC Is there some way to figure out which file(s)
are affected, and if they are stuff I don't care about, is there some
way to force btrfs to lose the 1gb it can't copy off of the failing
hdd?
Of course that's the classic raid0 trap (with btrfs multi-device single
being effectively a raid0 with really big stripes). Raid0 is (ideally)
never supposed to be used for data that isn't throw-away, either because
it's literally no-care data, or because there's backups kept
appropriately updated, as it's generally considered as good as dead the
moment one device fails or even really starts to go bad.
So ideally, with one device starting to go bad, you scrap the entire
filesystem, remove the bad device (or trigger sector remap and reuse, but
that's dangerous as once sectors start to go, generally the badness
spreads so the entire device can't be considered trustworthy again), and
mkfs a new filesystem on the remaining devices, with a replacement device
thrown in as well if desired.
But sometimes the world isn't ideal; on the arguably more practical
side... Most of my btrfs are raid1, both data/metadata, with the
remainder being mixed-bg dup, so I've never tried this on single,
personally, but...
First, you didn't mention versions so be sure you're current, btrfs-progs
v4.1.2 is current on the user side, kernel 4.1.x (which you appear to
have, based on the dmesg, BTW, gentoo here too =:^), or 4.2-rc5+ since
4.2 is close to release now, is current on the kernel side.
Try btrfs scrub. Assuming a current btrfs-progs, that should correct
errors in the metadata, which should be raid1 and thus have a second
hopefully valid copy to read from. It should detect but not be able to
correct errors in the single mode data, but should tell you what files
the errors are in (I believe very old btrfs-progs scrub did not).
Armed with a list of the files with errors, you should be able to delete
them. Once all such files are deleted, the 1 GiB chunk that they were in
should be empty, and a btrfs balance -dusage=0 should eliminate it.
At that point a btrfs dev del should work.
That's the theory, anyway. As I said, I've not tried it myself. But
it's what I'd try if I did have single-mode data on anything and found
myself in that situation.
--
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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