ps axl | grep D

F   UID   PID  PPID PRI  NI    VSZ   RSS WCHAN  STAT TTY        TIME COMMAND
4   104  2579  9109  20   0   5024  2116 refrig D    ?          0:00 cleanup -z -t unix -u -c
4   104  2842  9109  20   0   5016  2088 refrig D    ?          0:00 cleanup -z -t unix -u -c
4   104  3006  9109  20   0   5024  2092 refrig D    ?          0:00 cleanup -z -t unix -u -c
0 10022  4547  4865  20   0   4796  1536 refrig D    ?          0:00 /usr/sbin/postdrop -r
0 10006  4557  4865  20   0   4796  1544 refrig D    ?          0:00 /usr/sbin/postdrop -r

Detailed backup logs:

vzdump --quiet --node 2 --snapshot --compress --dumpdir /vz/backups --mailto xxx@xxxxxxxxx.com 101

101: Feb 03 01:00:04 INFO: Starting Backup of VM 101 (openvz)
101: Feb 03 01:00:04 INFO: CTID 101 exist mounted running
101: Feb 03 01:00:04 INFO: status = CTID 101 exist mounted running
101: Feb 03 01:00:04 INFO: mode failure - unable to detect lvm volume group
101: Feb 03 01:00:04 INFO: trying 'suspend' mode instead
101: Feb 03 01:00:04 INFO: backup mode: suspend
101: Feb 03 01:00:04 INFO: bandwidth limit: 10240 KB/s
101: Feb 03 01:00:04 INFO: starting first sync /var/lib/vz/private/101/ to /vz/vztmp/vzdumptmp4125
101: Feb 03 02:21:37 INFO: file has vanished: "/var/lib/vz/private/101/home/www/webx/user/webx_xxx/Maildir/courierimapkeywords/1265157441.15611_0.ns1"
101: Feb 03 03:29:29 INFO: Number of files: 414584
101: Feb 03 03:29:29 INFO: Number of files transferred: 385141
101: Feb 03 03:29:29 INFO: Total file size: 68275699091 bytes
101: Feb 03 03:29:29 INFO: Total transferred file size: 68273214154 bytes
101: Feb 03 03:29:29 INFO: Literal data: 68306670027 bytes
101: Feb 03 03:29:29 INFO: Matched data: 0 bytes
101: Feb 03 03:29:29 INFO: File list size: 8856510
101: Feb 03 03:29:29 INFO: File list generation time: 0.001 seconds
101: Feb 03 03:29:29 INFO: File list transfer time: 0.000 seconds
101: Feb 03 03:29:29 INFO: Total bytes sent: 68340236533
101: Feb 03 03:29:29 INFO: Total bytes received: 7447274
101: Feb 03 03:29:29 INFO: sent 68340236533 bytes  received 7447274 bytes  7623410.16 bytes/sec
101: Feb 03 03:29:29 INFO: total size is 68275699091  speedup is 1.00
101: Feb 03 03:29:29 INFO: rsync warning: some files vanished before they could be transferred (code 24) at main.c(1058) [sender=3.0.3]
101: Feb 03 03:29:29 INFO: first sync finished (8965 seconds)
101: Feb 03 03:29:29 INFO: suspend vm
101: Feb 03 03:29:29 INFO: Setting up checkpoint...
101: Feb 03 03:29:29 INFO: 	suspend...
101: Feb 03 03:29:39 INFO: Can not suspend container: Interrupted system call
101: Feb 03 03:29:39 INFO: Error: interrupted or timed out.
101: Feb 03 03:29:39 INFO: Checkpointing failed
101: Feb 03 03:37:51 ERROR: Backup of VM 101 failed - command 'vzctl --skiplock chkpnt 101 --suspend' failed with exit code 16

 ps aux | grep D
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root     22753  0.0  0.0   3104   688 pts/1    S+   09:35   0:00 grep D
postfix  23755  0.0  0.0   5024  2080 ?        D    03:29   0:00 cleanup -z -t unix -u -c