[libvirt-users] Why are virsh vol-upload/vol-download so slow?

[Lars Kellogg-Stedman]

I was interested in using 'virsh vol-upload' and 'virsh vol-download',
rather than mucking about directly with the filesystem.  But while
simply copying the image is relatively quick:

  # ls -l /var/lib/libvirt/images/centos-7-cloud.qcow2
  -rw-r--r--. 1 qemu qemu 1004994560 Apr 15  2015 centos-7-cloud.qcow2
  # time cp /var/lib/libvirt/images/centos-7-cloud.qcow2 /tmp/myimage.qcow2
  real	0m10.829s
  user	0m0.024s
  sys	0m1.668s

Trying to do the same thing with `virsh vol-download` is glacial:

  # time virsh vol-download --pool default centos-7-cloud.qcow2 /tmp/myimage.qcow2

I killed it after 10 minutes:

  real	10m20.775s
  user	10m19.451s
  sys	0m1.887s

After which is was about halfway through:

  # ls -l /tmp/myimage.qcow2
  -rw-r--r--. 1 root root 564920248 Mar  4 11:11 /tmp/myimage.qcow2

What is up with that?

-- 
Lars Kellogg-Stedman <lars at redhat.com> | larsks @ {freenode,twitter,github}
Cloud Engineering / OpenStack          | http://blog.oddbit.com/

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