[Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive

[Kevin Wolf <kwolf@redhat.com>]

Am 12.05.2010 17:05, schrieb Alexander Graf:
> Kevin Wolf wrote:
>> Am 10.05.2010 23:51, schrieb Alexander Graf:
>>   
>>> Usually the guest can tell the host to flush data to disk. In some cases we
>>> don't want to flush though, but try to keep everything in cache.
>>>
>>> So let's add a new parameter to -drive that allows us to set the flushing
>>> behavior to "on" or "off", defaulting to enabling the guest to flush.
>>>
>>> Signed-off-by: Alexander Graf <agraf@suse.de>
>>>     
>>
>> What about another cache=... value instead of adding more options? I'm
>> quite sure you'll only ever need this with writeback caching. So we
>> could have cache=none|writethrough|writeback|wb-noflush or something
>> like that.
>>
>>   
> 
> Yes, cache=volatile seems reasonable. Or cache=unsafe.
> 
>>> ---
>>>  block/raw-posix.c |   13 +++++++++++++
>>>     
>>
>> This is obviously wrong. If you want to introduce new behaviour to the
>> block layer, you must do it consistently and not just for one block
>> driver. So these changes should be made to the generic functions in
>> block.c instead.
>>   
> 
> How so? The callback functions are called using bdrv->drv->xxx. If I
> modify that pointer, I end up affecting all other virtual disks as well.

By doing the check in bdrv_flush/bdrv_aio_flush before this function
pointer is even called.

Kevin