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 <address@hidden>
raw-win32.c probably needs to have a .has_variable_length=true in
bdrv_host_device. Apart from that,
Reviewed-by: Paolo Bonzini <address@hidden>