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

[Jamie Lokier]

Kevin Wolf wrote:
> Am 08.01.2013 11:39, schrieb Liu Yuan:
> > On 01/08/2013 06:00 PM, Kevin Wolf wrote:
> >> Am 08.01.2013 10:45, schrieb Liu Yuan:
> >>> On 01/08/2013 05:40 PM, Stefan Hajnoczi wrote:
> >>>> Otherwise use sheepdog writeback and let QEMU block.c decide when to
> >>>> flush.  Never use sheepdog writethrough because it's redundant here.
> >>>
> >>> I don't get it. What do you mean by 'redundant'? If we use virtio &
> >>> sheepdog block driver, how can we specify writethrough mode for Sheepdog
> >>> cache? Here 'writethrough' means use a pure read cache, which doesn't
> >>> need flush at all.
> >>
> >> A writethrough cache is equivalent to a write-back cache where each
> >> write is followed by a flush. qemu makes sure to send these flushes, so
> >> there is no need use Sheepdog's writethrough mode.
> > 
> > Implement writethrough as writeback + flush will cause considerable
> > overhead for network block device like Sheepdog: a single write request
> > will be executed as two requests: write + flush
> 
> Yeah, maybe we should have some kind of a FUA flag with write requests
> instead of sending a separate flush.

Note that write+FUA has different semantics than write+flush, at least
with regular disks.

write+FUA commits just what was written, while write+flush commits
everything that was written before.

-- Jamie