Speeding up rbd_stat() in libvirt

[Wido den Hollander <wido@42on.com>]

Hi,

The storage pools of libvirt know a mechanism called 'refresh' which
will scan a storage pool to refresh the contents.

The current implementation does:
* List all images via rbd_list()
* Call rbd_stat() on each image

Source:
http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/storage/storage_backend_rbd.c;h=cdbfdee98505492407669130712046783223c3cf;hb=master#l329

This works, but a RBD pool with 10k images takes a couple of minutes to
scan.

Now, Ceph is distributed, so this could be done in parallel, but before
I start on this I was wondering if somebody had a good idea to fix this?

I don't know if it is allowed in libvirt to spawn multiple threads and
have workers do this, but it was something which came to mind.

libvirt only wants to know the size of a image and this is now stored in
the rbd_directory object, so the rbd_stat() is required.

Suggestions or ideas? I would like to have this process to be as fast as
possible.

Wido