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

[Liu Yuan]

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