On 23/04/2015 11:14, Wen Congyang wrote: > The bs->file->driver should support backing file, and use backing reference > already. > > What about the primary side? We should control when to connect to NBD server, > not in nbd_open().
My naive suggestion could be to add a BDRV_O_NO_CONNECT option to bdrv_open and a separate bdrv_connect callback. Open would fail if BDRV_O_NO_CONNECT is specified and drv->bdrv_connect is NULL. You would then need a way to have quorum pass BDRV_O_NO_CONNECT. Perhaps quorum is not a great match after all, and it's better to add a new "colo" driver similar to quorum but simpler and only using the read policy that you need for colo. The new driver would also know how to use BDRV_O_NO_CONNECT. In any case the amount of work needed would not be too big. Paolo
