Am 29.10.2013 um 13:02 hat Paolo Bonzini geschrieben: > Il 29/10/2013 12:35, Kevin Wolf ha scritto: > > The block layer generally keeps the size of an image cached in > > bs->total_sectors so that it doesn't have to perform expensive > > operations to get the size whenever it needs it. > > > > This doesn't work however when using a backend that can change its size > > without qemu being aware of it, i.e. passthrough of removable media like > > CD-ROMs or floppy disks. For this reason, the caching is disabled when a > > removable device is used. > > > > It is obvious that checking whether the _guest_ device has removable > > media isn't the right thing to do when we want to know whether the size > > of the host backend can change. To make things worse, non-top-level > > BlockDriverStates never have any device attached, which makes qemu > > assume they are removable, so drv->bdrv_getlength() is always called on > > the protocol layer. In the case of raw-posix, this causes unnecessary > > lseek() system calls, which turned out to be rather expensive. > > > > This patch completely changes the logic and disables bs->total_sectors > > caching only for certain block driver types, for which a size change is > > expected: host_cdrom and host_floppy; also the raw format in case it > > sits on top of one of these protocols, but in the common case the nested > > bdrv_getlength() call on the protocol driver will use the cache again > > and avoid an expensive drv->bdrv_getlength() call. > > > > Signed-off-by: Kevin Wolf <kw...@redhat.com>
> raw-win32.c probably needs to have a .has_variable_length=true in > bdrv_host_device. Apart from that, > > Reviewed-by: Paolo Bonzini <pbonz...@redhat.com> Thanks, good catch. I've added this now. Kevin