/dev/vg/somevolume: read failed after 0 of 4096 at nnnnn: Input/output error
If you’ve ever seen the above error, this usually means you have run out of disk space on the CoW-volume of a snapshot volume.
…but there is another uses for snapshots, and that is thin provisioning for sparse data use. If you create an LVM volume using the –virtualsize option, you can provide a logical size that is much larger than the actual underlying volume. If you exceed the space for such a volume, you will get the same error above—and all data on the volume will be invalidated and inaccessible.
LVM silently uses the ‘zero’ devicemapper target as the underlying volume. Thus, even though the data is invalidated nothing is lost. By overlaying the lost data over the top of a zero device, we can resurrect the data.
We have prepared our example file with the following:
lvcreate -L 100m --virtualsize 200m -n virtual_test vg
mount /dev/vg/virtual_test /mnt/tmp/
And now we fill the disk:
dd if=/dev/zero of=/mnt/tmp/overflow-file
dd: writing to `/mnt/tmp/overflow-file': Input/output error
Message from syslogd@backup at Aug 27 15:17:27 ...
kernel:journal commit I/O error
272729+0 records in
272728+0 records out
139636736 bytes (140 MB) copied
[I had to reboot here. The kernel still thought
the filesystem was mounted and I could not continue.
Obviously we are working near the kernel's limits on
this CentOS 6.2 2.6.32-based kernel]
Now we have a 200MB volume with 100MB allocated to it, which is now full. LVM has marked the volume as invalid and the data is no longer available.
First, resize the volume so we have room after resizing. Otherwise, the first byte written to the volume would, again, invalidate the disk:
lvresize -L +100m /dev/vg/virtual_test
[errors, possibly, just ignore them]
Extending logical volume virtual_test to 200.00 MiB
Logical volume virtual_test successfully resized
Now we edit the -cow file directly with a short perl script. The 5th byte is the ‘valid’ flag (see http://www.redhat.com/archives/linux-lvm/2006-September/msg00132.html) so all we need to is set it to ‘1’:
perl -e 'open(F, ">>", "/dev/mapper/vg-virtual_test-cow"); seek(F, 4, SEEK_SET); syswrite(F,"\x01",1); close(F);'
Now have lvm re-read the CoW metadata and you’re in business:
lvchange -an /dev/backup/virtual_test
lvchange -ay /dev/backup/virtual_test
[shouldn't have any errors]
LV VG Attr LSize Pool Origin Data%
virtual_test vg swi-a-s- 200.00m [virtual_test_vorigin] 33.63
At this point you should probably fsck your filesystem, it may be damaged—or at least nead a journal-replay since it stopped abruptly at the end of its allocated space. And as you can see, the “overflow” file is there up until the point of filling the disk.
[root@backup mapper]# e2fsck /dev/vg/virtual_test
e2fsck 1.41.12 (17-May-2010)
/dev/vg/virtual_test: recovering journal
/dev/vg/virtual_test: clean, 12/51200 files, 66398/204800 blocks
[root@backup mapper]# mount /dev/vg/virtual_test /mnt/tmp/
[root@backup mapper]# ls -lh /mnt/tmp/
drwx------. 2 root root 12K Aug 27 15:16 lost+found
-rw-r--r--. 1 root root 54M Aug 27 15:17 overflow-file