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

[MORITA Kazutaka]

At Thu, 10 Jan 2013 13:38:16 +0800,
Liu Yuan wrote:
> 
> On 01/09/2013 11:10 PM, Paolo Bonzini wrote:
> > Il 09/01/2013 14:04, Liu Yuan ha scritto:
> >>>>   2 The upper layer software which relies on the 'cache=xxx' to choose
> >>>> cache mode will fail its assumption against new QEMU.
> >>>
> >>> Which assumptions do you mean? As far as I can say the behaviour hasn't
> >>> changed, except possibly for the performance.
> >>
> >> When users set 'cache=writethrough' to export only a writethrough cache
> >> to Guest, but with new QEMU, it will actually get a writeback cache as
> >> default.
> > 
> > They get a writeback cache implementation-wise, but they get a
> > writethrough cache safety-wise.  How the cache is implemented doesn't
> > matter, as long as it "looks like" a writethrough cache.
> > 
> 
> > In fact, consider a local disk that doesn't support FUA.  In old QEMU,
> > images used to be opened with O_DSYNC and that splits each write into
> > WRITE+FLUSH, just like new QEMU.  All that changes is _where_ the
> > flushes are created.  Old QEMU changes it in the kernel, new QEMU
> > changes it in userspace.
> > 
> >> We don't need to communicate to the guest. I think 'cache=xxx' means
> >> what kind of cache the users *expect* to export to Guest OS. So if
> >> cache=writethrough set, Guest OS couldn't turn it to writeback cache
> >> magically. This is like I bought a disk with 'writethrough' cache
> >> built-in, I didn't expect that it turned to be a disk with writeback
> >> cache under the hood which could possible lose data when power outage
> >> happened.
> > 
> > It's not by magic.  It's by explicitly requesting the disk to do this.
> > 
> > Perhaps it's a bug that the cache mode is not reset when the machine is
> > reset.  I haven't checked that, but it would be a valid complaint.
> > 
> 
> Ah I didn't get the current implementation right. I tried the 3.7 kernel
> and it works as expected (cache=writethrough result in a 'writethrough'
> cache in the guest).
> 
> It looks fine to me to emulate writethrough as writeback + flush, since
> the profermance drop isn't big, though sheepdog itself support true
> writethrough cache (no flush).

Can we drop the SD_FLAG_CMD_CACHE flag from sheepdog write requests
when bdrv_enable_write_cache() is false?  Then the requests behave
like FUA writes and we can safely omit succeeding flush requests.

Thanks,

Kazutaka