Re: [libvirt] [Qemu-devel] [RFC 0/5] block: File descriptor passing using -open-hook-fd

On 05/17/2012 07:42 AM, Stefan Hajnoczi wrote: snapshot_blkdev can take an fd:name instead of a /path/to/file for the file to open, in which case libvirt can pass in the named fd _prior_ to the snapshot_blkdev using the 'getfd' monitor command. The -open-hook-fd approach would indeed allow snapshot_blokdev to ask for the fd after the fact, but it's much more painful. Consider a case with a two-disk snapshot: with the fd:name approach, the sequence is: libvirt calls getfd:name1 over normal monitor qemu responds libvirt calls getfd:name2 over normal monitor qemu responds libvirt calls transaction around blockdev-snapshot-sync over normal monitor, using fd:name1 and fd:name2 qemu responds but with -open-hook-fd, the approach would be: libvirt calls transaction qemu calls open(file1) over hook libvirt responds qemu calls open(file2) over hook libvirt responds qemu responds to the original transaction The 'transaction' operation is thus blocked by the time it takes to do two intermediate opens over a second channel, which kind of defeats the purpose of making the transaction take effect with minimal guest downtime. And libvirt code becomes a lot trickier to deal with the fact that two channels are in use, and that the channel that issued the 'transaction' command must block while the other channel for handling hooks must be responsive. I'm really disliking the hook-fd approach, when a better solution is to make use of 'getfd' in advance of any operation that will need to open new fds. -- Eric Blake eblake(a)redhat.com +1-919-301-3266 Libvirt virtualization library http://libvirt.org