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Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics
From: |
Liu Yuan |
Subject: |
Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics |
Date: |
Sat, 05 Jan 2013 15:56:11 +0800 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0 |
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
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/03
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Stefan Hajnoczi, 2013/01/04
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/04
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Kevin Wolf, 2013/01/07
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/08
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Stefan Hajnoczi, 2013/01/08
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/08
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Kevin Wolf, 2013/01/08
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/08
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Kevin Wolf, 2013/01/08
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/08