On 28/02/2024 18.25, Patrick Plenefisch wrote:
I'm unsure if this is just an LVM bug, or a BTRFS+LVM interaction bug,
but LVM is definitely involved somehow.
Upgrading from 5.10 to 6.1, I noticed one of my filesystems was
read-only. In dmesg, I found:
BTRFS error (device dm-75): bdev /dev/mapper/lvm-brokenDisk errs: wr
0, rd 0, flush 1, corrupt 0, gen 0
BTRFS warning (device dm-75): chunk 13631488 missing 1 devices, max
tolerance is 0 for writable mount
BTRFS: error (device dm-75) in write_all_supers:4379: errno=-5 IO
failure (errors while submitting device barriers.)
BTRFS info (device dm-75: state E): forced readonly
BTRFS warning (device dm-75: state E): Skipping commit of aborted transaction.
BTRFS: error (device dm-75: state EA) in cleanup_transaction:1992:
errno=-5 IO failure
At first I suspected a btrfs error, but a scrub found no errors, and
it continued to be read-write on 5.10 kernels.
Here is my setup:
/dev/lvm/brokenDisk is a lvm-on-lvm volume. I have /dev/sd{a,b,c,d}
(of varying sizes) in a lower VG, which has three LVs, all raid1
volumes. Two of the volumes are further used as PV's for an upper VGs.
One of the upper VGs has no issues. The non-PV LV has no issue. The
remaining one, /dev/lowerVG/lvmPool, hosting nested LVM, is used as a
PV for VG "lvm", and has 3 volumes inside. Two of those volumes have
no issues (and are btrfs), but the last one is /dev/lvm/brokenDisk.
This volume is the only one that exhibits this behavior, so something
is special.
Or described as layers:
/dev/sd{a,b,c,d} => PV => VG "lowerVG"
/dev/lowerVG/single (RAID1 LV) => BTRFS, works fine
/dev/lowerVG/works (RAID1 LV) => PV => VG "workingUpper"
/dev/workingUpper/{a,b,c} => BTRFS, works fine
/dev/lowerVG/lvmPool (RAID1 LV) => PV => VG "lvm"
/dev/lvm/{a,b} => BTRFS, works fine
/dev/lvm/brokenDisk => BTRFS, Exhibits errors
I am a bit curious about the reasons of this setup. However I understood that:
/dev/sda -+ +-- single (RAID1) -> ok +-> a ok
/dev/sdb | | |-> b ok
/dev/sdc +--> [lowerVG]>--+-- works (RAID1) -> [workingUpper] -+-> c ok
/dev/sdd -+ |
| +-> a -> ok
+-- lvmPool -> [lvm] ->-|
+-> b -> ok
|
+->brokenDisk -> fail
[xxx] means VG, the others are LVs that may act also as PV in
an upper VG
So, it seems that
1) lowerVG/lvmPool/lvm/a
2) lowerVG/lvmPool/lvm/a
3) lowerVG/lvmPool/lvm/brokenDisk
are equivalent ... so I don't understand how 1) and 2) are fine but 3) is
problematic.
Is my understanding of the LVM layouts correct ?
After some investigation, here is what I've found:
1. This regression was introduced in 5.19. 5.18 and earlier kernels I
can keep this filesystem rw and everything works as expected, while
5.19.0 and later the filesystem is immediately ro on any write
attempt. I couldn't build rc1, but I did confirm rc2 already has this
regression.
2. Passing /dev/lvm/brokenDisk to a KVM VM as /dev/vdb with an
unaffected kernel inside the vm exhibits the ro barrier problem on
unaffected kernels.
Is /dev/lvm/brokenDisk *always* problematic with affected ( >= 5.19 ) and
UNaffected ( < 5.19 ) kernel ?
3. Passing /dev/lowerVG/lvmPool to a KVM VM as /dev/vdb with an
affected kernel inside the VM and using LVM inside the VM exhibits
correct behavior (I can keep the filesystem rw, no barrier errors on
host or guest)
Is /dev/lowerVG/lvmPool problematic with only "affected" kernel ?
[...]
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