Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics

[Liu Yuan <namei.unix@gmail.com>]

On 01/05/2013 01:29 PM, Liu Yuan wrote:
> On 01/05/2013 12:40 PM, Liu Yuan wrote:
>> On 01/05/2013 12:38 AM, Stefan Hajnoczi wrote:
>>> Hi Yuan,
>>> BDRV_O_NOCACHE means bypass host page cache (O_DIRECT).
>>>
>>> BDRV_O_CACHE_WB specifies the cache semantics that the guest sees - that
>>> means whether the disk cache is writethrough or writeback.
>>>
>>> In other words, BDRV_O_NOCACHE is a host performance tweak while
>>> BDRV_O_CACHE_WB changes the cache safety of the BlockDriverState.  A
>>> protocol driver like sheepdog doesn't need to look at BDRV_O_CACHE_WB
>>> because it is implemented in block.c (see bdrv_co_do_writev() where QEMU
>>> will flush when after each write when !bs->enable_write_cache).
>>
>> Hi Stefan,
>>
>>   Thanks for your explanation. But after more investigation, I find
>> myself more confused:
>>
>>                       flags passed from block layer
>>   {writeback, writethrough}      0x2042
>>   {directsync, off, none}        0x2062
>>   {unsafe}                       0x2242
>>
>> So underlying driver like Sheepdog can't depend on 'flags' passed from
>> .bdrv_file_open() to choose the right semantics (This was possible for
>> old QEMU IIRC).
>>
>> If we can't rely on the 'flags' to get the cache indications of users,
>> would you point me how to implement tristate cache control for network
>> block driver like Sheepdog? For e.g, I want to implement following
>> semantics:
>>     cache=writeback|none|off # enable writeback semantics for write
>>     cache=writethrough # enable writethrough semantics for write
>>     cache=directsync # disable cache completely
>>
>> Thanks,
>> Yuan
>>
> 
> I tried the old QEMU and v1.1.0 and v1.1.2, they still worked as I
> expected. So I guess generic block layer got changed a bit and the
> 'flags' meaning turned different than old code, which did indeed allow
> block drivers to interpret the 'flags' passed from bdrv_file_open().
> 
> With the current upstream code, it seems that BDRV_O_CACHE_WB is always
> enabled and makes 'flags' completely unusable for block drivers to get
> the indications of user by specifying 'cache=' field.
> 
> So is there other means to allow block drivers to rely on, in order to
> interpret the 'cache semantics'?
> 

I found the commit:e1e9b0ac 'always open drivers in writeback mode'.
This is really undesired for network block drivers such as Sheepdog,
which implement its own cache mechanism that support
writeback/writethrough/directio behavior and then want to interpret
these flags on its own.

Is there any means for block drivers to get the semantics of 'cache=xxx'
from users?

Thanks,
Yuan