Andreas,

LVM RAID and MD RAID have different, hence incompatible superblock formats
so you can't switch to MD in this case.

Try activating your RaidLV with 'lvchange -ay --activationmode=degraded
/dev/vg_ssds_0/host-home', add a replacement PV of adequate size if none
available and run 'lvconvert --repair /dev/vg_ssds_0/host-home'.

Best,
Heinz

On Fri, Oct 29, 2021 at 1:07 PM Andreas Trottmann <
andreas.trottmann@werft22.com> wrote:

> Am 11.10.21 um 16:08 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
>
>
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