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

[Fam Zheng <famz@redhat.com>]

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).

Fam