Re: [Qemu-devel] [PATCH] Align file accesses with cache=off (O_DIRECT)

[Kevin Wolf <kwolf@suse.de>]

Kevin Wolf schrieb:
> Am Donnerstag, 1. Mai 2008 16:35:51 schrieb Blue Swirl:
>> On 5/1/08, Kevin Wolf <mail@kevin-wolf.de> wrote:
>>> Am Mittwoch, 30. April 2008 16:30:27 schrieb Blue Swirl:
>>>> Maybe the alignment could be handled like AIO and synchronous IO
>>>>
>>>  > emulation layers are added in bdrv_register, but at open stage?
>>>
>>> You mean to preserve the original pread if the file is opened without
>>> O_DIRECT and replace it by the emulation function only if O_DIRECT is
>>> really used?
>> Right.
>>
>> Maybe it's slightly faster that way and it would be closer to how
>> other block emulations are handled. It's just an idea.
> 
> Maybe I'm missing something but AFAIK this pread pointer exists once for each 
> block driver, i.e. every raw image uses the original pread or every raw image 
> uses the emulating one. The difference between the O_DIRECT case and the 
> AIO/sync emulation is that AIO/sync is the same for all devices of one driver 
> while O_DIRECT can differ between images of the same driver.
> 
> So you would need to have one common pread which in turn calls a function 
> pointer stored in the BlockDriverState. I doubt that this is much cheaper 
> than an if in pread. And it wouldn't get too close to other emulations anyway 
> because of the driver/device difference.

Should I change the patch now (even if I think it doesn't help anything) 
or will you apply the patch as it is? It is quite frustrating to get no 
answer at all.

Kevin