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

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

On 01/07/2013 08:31 PM, Stefan Hajnoczi wrote:
> On Sat, Jan 05, 2013 at 03:56:11PM +0800, Liu Yuan wrote:
>> 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?
> 
> Hi Yuan,
> Please explain what cache semantics the sheepdog server supports so I
> can understand better what you are trying to achieve.
> 

Hi Stefan,

Sheepdog support writethrough/writeback/directio(bypass) the cache, and
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