linux-kernel - Re: [Xen-devel] Re: [PATCH] xen block backend driver.

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Date:	Thu, 21 Apr 2011 15:06:06 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Daniel Stodden <daniel.stodden@...rix.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"jaxboe@...ionio.com" <jaxboe@...ionio.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"konrad@...nel.org" <konrad@...nel.org>
Subject: Re: [Xen-devel] Re: [PATCH] xen block backend driver.

On Thu, Apr 21, 2011 at 12:03:12PM -0700, Daniel Stodden wrote:
> Yes, everybody is aware that the semantics were broken. But note it's
> not even a consistency issue at this point, because there's currently no
> frontend which relies on the original ordering semantics either. Take
> xen-blkfront, since blk_flush it uses the barrier op for a flush, being
> just a superset when ordering is enforced.

There is a huge userbase of guests out there that does rely on it.

> But before we just enumerate a new command, a potentially more viable
> option would be FLUSH+FUA flags on the WRITE operation. As if mapping
> bio bits.
> 
> The advantage is that it avoids the extra round trip implied by having
> the frontend driving writes through FSEQ_PREFLUSH on their own. I'd
> expect that to make much more of a performance difference. Somewhat
> differentiating PV from the low physical layer.
> 
> Would you, maybe did you, consider that? I think it sounds interesting
> enough to gather performance data, just asking beforehand.

You will need a pure flush anyway.  Once you actually have a correct
implementation you can look into optimizing it.  Note that at least
the Solaris Xen coded added a cache flush to the protocol.

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