On 03/14/2012 07:19 AM, Alon Levy wrote:

>> Also, when returning the image via fd, we could offload its writing to
>> a bh. This would give us a poor man's async support, where the biggest
>> drawback would the lack of error detection (can this operation fail anyway?).
>>
> 
> It leaves detection of completion up to the user, so inotify / parsing
> the file to see if it's complete (header says size X, but file is still
> Y<X, not done..), which looks like as big a drawback.

Notification on a pipe is easy - the reader gets EOF when the writer
closes the fd, with no need to use inotify.  The only remaining issue is
whether the writer closes the fd early on error, so that the reader gets
an incomplete file.  But then the client can tell after getting EOF
whether the image file is complete (read the header, final file size is
too small), without having to have the intermediate management app
parsing headers during the streaming, although it would be a bonus if
there were a way to tell the management app that an error occurred and
qemu closed the write end early.

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org