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I have a filesystem mounted with 9p virtio through KVM, and am backing it up using duplicity to a remote SSH server. I' m trying to speed up the backup process, which seems unreasonably slow to me.

The source size is 20GB in 107.651 files, which are on an ext4 filesystem on the virtual machine host running Ubuntu 14.04, on top of a Raid10 array on a 3ware controller using 15K disks (WD VelociRaptors), no BBWC. The virtual machine itself is Ubuntu 12.04.5 mounting the files with p9 over virtio, driver "path", mode "mapped", write policy "immediate". The destination over SSH is a HP server with 512MB BBWC enabled with 12x 2TB SAS disks, confirmed to be blazingly fast.

If all else fails, I'll just try the duplicity run on the virtual machine host to eliminate the 9p middle layer in accessing the files in order to see if 9p is the issue (which I'm slowly suspecting it is)

Here are the duplicity backup statistics:

--------------[ Backup Statistics ]--------------
StartTime 1483275839.07 (Sun Jan  1 14:03:59 2017)
EndTime 1483332365.62 (Mon Jan  2 05:46:05 2017)
ElapsedTime 56526.55 (15 hours 42 minutes 6.55 seconds)
SourceFiles 107651
SourceFileSize 21612274293 (20.1 GB)
NewFiles 24
NewFileSize 69952 (68.3 KB)
DeletedFiles 11
ChangedFiles 38
ChangedFileSize 6825600 (6.51 MB)
ChangedDeltaSize 0 (0 bytes)
DeltaEntries 73
RawDeltaSize 47509 (46.4 KB)
TotalDestinationSizeChange 103051 (101 KB)
Errors 0

The python cProfile run returned the following functions took the longest execution time:

29225254 function calls (29223127 primitive calls) in 56578.118 seconds
   ncalls   tottime  percall   cumtime  percall filename:lineno(function)
   107700 28238.712    0.262 28238.712    0.262 {posix.lstat}
   107650 28016.367    0.260 28016.367    0.260 {posix.access}
      892   190.827    0.214   190.827    0.214 {posix.listdir}
        2    49.552   24.776    49.552   24.776 {method 'readline' of 'file' objects}
       82    11.113    0.136    11.113    0.136 {open}

1 Answer 1

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9p is the issue. Running duplicity on the VM host, where the data is located, it was done in 55 seconds.

This bug is apparently still open, which talks about the same performance issues. It suggests adding msize=262144 to the mount options, which does speed up the access a bit, but it's still nowhere near as fast as direct access.

So in conclusion, don't use 9p over virtio and expect high file access speed. In my case, the application which accesses these files over 9p is not affected much, but others (like duplicity) are.

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