VZDump backup failed

For the last few weeks, my VZDump backups have been failing:

Feb 07 22:56:44 INFO: Starting Backup of VM 100 (qemu)
Feb 07 22:56:44 INFO: status = running
Feb 07 22:56:45 INFO: backup mode: snapshot
Feb 07 22:56:45 INFO: ionice priority: 7
Feb 07 22:56:45 INFO: creating archive '/backup/dump/vzdump-qemu-100-2013_02_07-22_56_44.vma.lzo'
Feb 07 22:56:45 INFO: started backup task '4c6705b2-8010-4cbd-819a-2c6c67e76f62'
Feb 07 22:56:48 INFO: status: 0% (369426432/107374182400), sparse 0% (40050688), duration 3, 123/109 MB/s
Feb 07 22:56:57 INFO: status: 1% (1186594816/107374182400), sparse 0% (85319680), duration 12, 90/85 MB/s
Feb 07 22:57:06 INFO: status: 2% (2167275520/107374182400), sparse 0% (301617152), duration 21, 108/84 MB/s
Feb 07 22:57:17 INFO: status: 3% (3300065280/107374182400), sparse 0% (377200640), duration 32, 102/96 MB/s
Feb 07 22:57:30 INFO: status: 3% (4280025088/107374182400), sparse 0% (410435584), duration 45, 75/72 MB/s
Feb 07 22:57:30 INFO: aborting backup job
Feb 07 22:57:31 ERROR: Backup of VM 100 failed - vma_co_write write error - Broken pipe

A quick search for vma_co_write write error - Broken pipe only found the source code that writes the error :)

dmesg does not show i/o error on either source lv/vg/pv or physical device, or backup target. No other strange system log errors either.