[fedora-virt] porting a disk from lvm to file

[Richard W.M. Jones]

On Sun, Aug 22, 2010 at 07:28:50PM +0300, Avi Kivity wrote:
>  On 08/01/2010 11:41 PM, Richard W.M. Jones wrote:
> >On Wed, Jul 28, 2010 at 07:39:30PM +0200, Sven Lankes wrote:
> >>You might want to add -c to the ssh cmd to compress the data if the
> >>connection between the servers is<  100 MBit/s.
> >When writing the original virt-p2v we found the -C option actually
> >makes things a lot slower, assuming the common case where you have a
> >gigabit network between the servers.  It's quicker just to copy
> >without the overhead of compression.
> 
> Interesting, I'd have guessed that encryption will dominate the cpu
> cost, and that compression would be a win since there's less to
> encrypt and transmit.

Maybe my explanation is wrong too.  virt-p2v was definitely much
slower when we added the '-C' option.  However read on.

I just ran a test again on my local LAN.  This is between two
approximately equal Fedora machines, over a moderate quality consumer
gigabit ethernet switch.  The command approximates what virt-p2v does:
sending 1MB blocks from local /dev device, and at the target end using
cat to write to a file.

$ time sh -c 'dd bs=1M if=/dev/vg_trick/Windows7x64 | ssh amd "cat > /tmp/copy1"'
16384+0 records in
16384+0 records out
17179869184 bytes (17 GB) copied, 1473.26 s, 11.7 MB/s

real	    24m33.269s
user	    4m16.944s
sys	    4m43.181s

$ time sh -c 'dd bs=1M if=/dev/vg_trick/Windows7x64 | ssh -C amd "cat > /tmp/copy2"'
16384+0 records in
16384+0 records out
17179869184 bytes (17 GB) copied, 1412.7 s, 12.2 MB/s

real	    23m32.736s
user	    17m52.739s
sys	    5m0.884s

In summary:

Copy rate (no compression):   11.7 MB/s
Copy rate (with compression): 12.2 MB/s

So now compression is (slightly) faster.  YMMV.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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