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

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

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'?

Thanks,
Yuan