Backup hangs part way through

[philled]

I have a Ubuntu-based VM which hangs every time I try to back it up. The VM's disk format is 10GB raw, it has 1 CPU and 2GB RAM, virtio Network. I'm backing up to an NFS share on a QNAP NAS on the same subnet. I'm running Proxmox 3.4-1/3f2d890e.

Here is all of the backup output from the last attempt when I ran it from the Proxmox host command line:

INFO: starting new backup job: vzdump 100 --dumpdir /mnt/nas_phill/Backup/VirtualMachines --mailto myname@example.com --mode stop --compress gzip --maxfiles 3
INFO: Starting Backup of VM 100 (qemu)
INFO: status = stopped
INFO: update VM 100: -lock backup
INFO: backup mode: stop
INFO: ionice priority: 7
INFO: creating archive '/mnt/nas_phill/Backup/VirtualMachines/vzdump-qemu-100-2015_06_08-23_13_58.vma.gz'
INFO: starting kvm to execute backup task
INFO: started backup task '31b83deb-2a14-4da4-8175-730e3c2f14f8'
INFO: status: 1% (110231552/10737418240), sparse 0% (55889920), duration 3, 36/18 MB/s
INFO: status: 2% (224264192/10737418240), sparse 0% (65855488), duration 9, 19/17 MB/s
INFO: status: 3% (334495744/10737418240), sparse 0% (66498560), duration 15, 18/18 MB/s
INFO: status: 4% (437125120/10737418240), sparse 0% (67723264), duration 21, 17/16 MB/s
INFO: status: 5% (551157760/10737418240), sparse 0% (85438464), duration 27, 19/16 MB/s
INFO: status: 6% (649986048/10737418240), sparse 0% (91672576), duration 33, 16/15 MB/s
INFO: status: 7% (752615424/10737418240), sparse 0% (93126656), duration 39, 17/16 MB/s
INFO: status: 8% (859045888/10737418240), sparse 0% (93138944), duration 46, 15/15 MB/s
INFO: status: 9% (973078528/10737418240), sparse 0% (93323264), duration 54, 14/14 MB/s
INFO: status: 10% (1083310080/10737418240), sparse 0% (93507584), duration 62, 13/13 MB/s
INFO: status: 11% (1185939456/10737418240), sparse 0% (93507584), duration 69, 14/14 MB/s
INFO: status: 12% (1296171008/10737418240), sparse 0% (94552064), duration 76, 15/15 MB/s
INFO: status: 13% (1414004736/10737418240), sparse 0% (97779712), duration 82, 19/19 MB/s
INFO: status: 14% (1538392064/10737418240), sparse 0% (105041920), duration 85, 41/39 MB/s
INFO: status: 15% (1615462400/10737418240), sparse 1% (107458560), duration 88, 25/24 MB/s
INFO: status: 16% (1718091776/10737418240), sparse 1% (107520000), duration 95, 14/14 MB/s
INFO: status: 17% (1832124416/10737418240), sparse 1% (107548672), duration 102, 16/16 MB/s
INFO: status: 18% (1944518656/10737418240), sparse 1% (122744832), duration 107, 22/19 MB/s
INFO: status: 19% (2041184256/10737418240), sparse 1% (122744832), duration 111, 24/24 MB/s
INFO: status: 20% (2166358016/10737418240), sparse 1% (130015232), duration 118, 17/16 MB/s
INFO: status: 21% (2261647360/10737418240), sparse 1% (146534400), duration 122, 23/19 MB/s
INFO: status: 22% (2375680000/10737418240), sparse 1% (146882560), duration 128, 19/18 MB/s
INFO: status: 23% (2478309376/10737418240), sparse 1% (146952192), duration 133, 20/20 MB/s
INFO: status: 24% (2580938752/10737418240), sparse 1% (146976768), duration 139, 17/17 MB/s
INFO: status: 25% (2691170304/10737418240), sparse 1% (148230144), duration 146, 15/15 MB/s
INFO: status: 26% (2805202944/10737418240), sparse 1% (148828160), duration 151, 22/22 MB/s
INFO: status: 27% (2926837760/10737418240), sparse 1% (148959232), duration 155, 30/30 MB/s
INFO: status: 28% (3025666048/10737418240), sparse 1% (158138368), duration 159, 24/22 MB/s
INFO: status: 29% (3135897600/10737418240), sparse 1% (158343168), duration 164, 22/22 MB/s
INFO: status: 30% (3223322624/10737418240), sparse 1% (158343168), duration 168, 21/21 MB/s
INFO: status: 31% (3341156352/10737418240), sparse 1% (158384128), duration 175, 16/16 MB/s

There doesn't seem to be any errors anywhere or any hints about why it's hanging. I have another VM which backed up just fine which has qcow2 file system. Any suggestions about how I can fix this backup issue?

 

[philled]

I have some more info on this. I tried re-running the backup and using local storage on the Proxmox host as the dumpdir. This allowed the backup to complete. But it doesn't explain why this backup failed when dumpdir was an CIFS share on a NAS, but a backup of another VM to the same NFS share succeeded. There are no error messages or anything so it's very difficult to figure out what the problem is.

Obviously backing up to a local disk on the Proxmox host is not ideal so does anyone have any suggestions?