Am 08.10.21 um 21:57 schrieb Andreas Trottmann:
I am running a server that runs a number of virtual machines and manages
their virtual disks as logical volumes using lvmraid (...)
After a restart, all of the logical volumes came back, except one.
When I'm trying to activate it, I get:
# lvchange -a y /dev/vg_ssds_0/host-home
Couldn't find device with uuid 8iz0p5-vh1c-kaxK-cTRC-1ryd-eQd1-wX1Yq9.
device-mapper: reload ioctl on (253:245) failed: Input/output error
I am replying to my own e-mail here in order to document how I got the
data back, in case someone in a similar situation finds this mail when
searching for the symptoms.
First: I did *not* succeeed in activating the lvmraid volume. No matter
how I tried to modify the _rmeta volumes, I always got "reload ioctl
(...) failed: Input/output error" from "lvchange", and "cannot start
dirty degraded array" in dmesg.
So, I used "lvchange -a y /dev/vg_ssds_0/host-home_rimage_0" (and
_rimage_2 and _rimage_3, as those were the ones that were *not* on the
failed PV) to get access to the indivdual RAID SubLVs. I then used "dd
if=/dev/vg_ssds_0/host-home_rimage_0 of=/mnt/space/rimage_0" to copy the
data to a file on a filesystem with enough space. I repeated this with 2
and 3 as well. I then used losetup to access /mnt/space/rimage_0 as
/dev/loop0, rimage_2 as loop2, and rimage_3 as loop3.
Now I wanted to use mdadm to "build" the RAID in the "array that doesn't
have per-device metadata (superblocks)" case:
# mdadm --build /dev/md0 -n 4 -c 128 -l 5 --assume-clean --readonly
/dev/loop0 missing /dev/loop2 /dev/loop3
However, this failed with "mdadm: Raid level 5 not permitted with --build".
("-c 128" was the chunk size used when creating the lvmraid, "-n 4" and
"-l 5" refer to the number of devices and the raid level)
I then read the man page about the "superblocks", and found out that the
"1.0" style of RAID metadata (selected with an mdadm "-e 1.0" option)
places a superblock at the end of the device. Some experimenting on
unused devices showed that the size used for actual data was the size of
the block device minus 144 KiB (possibly 144 KiB = 128 KiB (chunksize) +
8 KiB (size of superblock) + 8 KiB (size of bitmap). So I added 147456
zero bytes at the end of each file:
# for i in 0 2 3; do head -c 147456 /dev/zero >> /mnt/space/rimage_$i; done
After detaching and re-attaching the loop devices, I ran
# mdadm --create /dev/md0 -n 4 -c 128 -l 5 -e 1.0 --assume-clean
/dev/loop0 missing /dev/loop2 /dev/loop3
(substituting "missing" in the place where the missing RAID SubLV would
have been)
And, voilà: /dev/md0 was perfectly readable, fsck showed no errors, and
it could be mounted correctly, with all data intact.
Kind regards
--
Andreas Trottmann
Werft22 AG
Tel +41 (0)56 210 91 32
Fax +41 (0)56 210 91 34
Mobile +41 (0)79 229 88 55
