[linux-lvm] Issues with md RAID-1, LVM, and snapshots

[Kevin Murphy]

I'm having trouble with snapshot volumes when combining software RAID-1
with LVM2 striping.

I have a server with four 18GB drives. I would like to combine them into
two RAID-1 mirrors and then stripe across them with LVM, with the  goal
of data redundancy and the ability to make hot backups via snapshots.

System:
Suse Linux Enterprise Server 9 for x86_64
kernel 2.6.5-7.147-smp
lvm2-2.00.15-0.8

When I set up the drives in one big volume group (no RAID), snapshots
work correctly:

pvcreate /dev/sdc1
pvcreate /dev/sdd1
pvcreate /dev/sdf1
pvcreate /dev/sde1
vgcreate -M2 vg1 /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1
lvcreate -i4 -I64 -L32G -n lv1 vg1
mkfs.ext2 /dev/vg1/lv1
mount /dev/vg1/lv1 /mnt/bigdisk
{put some data in /mnt/bigdisk}
lvcreate -L1G -s -n snap1 /dev/vg1/lv1
mount /dev/vg1/snap1 /mnt/backup
find /mnt/backup | cpio -ovHnewc > /tmp/backup.cpio

This works fine. However, if I set up two RAID-1 volumes and stripe
across them with LVM, I get errors when reading from a snapshot:

mdadm --create -l1 -n2 /dev/sdc1 /dev/sdd1 /dev/md2
mdadm --create -l1 -n2 /dev/sde1 /dev/sdf1 /dev/md3
pvcreate /dev/md2
pvcreate /dev/md3
vgcreate -M2 vg1 /dev/md2 /dev/md3
lvcreate -i2 -I64 -L32G -n lv1 vg1
mkfs.ext2 /dev/vg1/lv1
mount /dev/vg1/lv1 /mnt/bigdisk
{put some data in /mnt/bigdisk}
lvcreate -L1G -s -n snap1 /dev/vg1/lv1
mount /dev/vg1/snap1 /mnt/backup

I get this error:
cpio: Read error at byte 2129920 in file /mnt/backup/file1, padding with
zeros

dmesg says:
attempt to access beyond end of device
dm-3: rw=0, want=17060337672, limit=67108864
attempt to access beyond end of device
dm-3: rw=0, want=134226096, limit=67108864
attempt to access beyond end of device
dm-3: rw=0, want=3499657224, limit=67108864
attempt to access beyond end of device
dm-3: rw=0, want=3094910976, limit=67108864
attempt to access beyond end of device
dm-3: rw=0, want=8894171864, limit=67108864
attempt to access beyond end of device
dm-3: rw=0, want=17060337672, limit=67108864

When I extract the archive, file1 is corrupted.

Is there any special magic that needs to be done with LVM+MD, or have I
hit a bug?

--
Kevin Murphy