Re: [Qemu-devel] [PATCH] block: Avoid unecessary drv->bdrv_getlength() calls

[Kevin Wolf <kwolf@redhat.com>]

Am 04.11.2013 um 08:24 hat Fam Zheng geschrieben:
> 
> 
> On Tue 29 Oct 2013 08:12:38 PM CST, Kevin Wolf wrote:
> >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 <kwolf@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 <pbonzini@redhat.com>
> >
> >Thanks, good catch. I've added this now.
> 
> This breaks VMDK because it can't read description file: buffer is
> empty in bdrv_probe, when the file size is <512 bytes. (for
> raw-posix).

Perhaps we should replace bs->total_sectors with bs->total_byts one
day... For now, how about introducing a bdrv_getlength_bytes() that
always calls the driver and never converts bytes to sectors?

Kevin