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

[Kevin Wolf <kwolf@redhat.com>]

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.

Kevin