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. Stefan