I just encountered this while making a tar backup on one of my virtual filesystems:
lost page write due to I/O error on xvdb1 end_request: I/O error, dev xvdb1, sector 33707688 Buffer I/O error on device xvdb1, logical block 4213461
Unfortunately, my fs suffered corruption.
Some folks, particularly Ubuntu users with kernel 3.2.x seem to say the answer is adding
to your ext3/ext4 mount options.
In my case I'm pretty sure an OOM error killed something critical to the backend on dom0.
Be sure to check your dom0's dmesg.
UPDATE Uhh yeah, something actually managed to overflow the dom0's fs. >.> This is what I was getting in the dom0 dmesg:
loop: Write error at byte offset 17820364800, length 4096.
for which there is very little discussion, but a quick df told me what was going on.