linux-kernel - Re: [uml-devel] [PATCH] um: ubd: Add REQ

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Message-ID: <20130819110917.GD5918@redhat.com>
Date:	Mon, 19 Aug 2013 12:09:17 +0100
From:	"Richard W.M. Jones" <rjones@...hat.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Richard Weinberger <richard@....at>,
	Al Viro <viro@...iv.linux.org.uk>,
	uml-devel <user-mode-linux-devel@...ts.sourceforge.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [uml-devel] [PATCH] um: ubd: Add REQ_FLUSH suppport

On Mon, Aug 19, 2013 at 12:13:14PM +0200, Geert Uytterhoeven wrote:
> Hi Richard,
> 
> On Mon, Aug 19, 2013 at 11:18 AM, Richard W.M. Jones <rjones@...hat.com> wrote:
> > On Sun, Aug 18, 2013 at 12:09:34AM +0200, Richard Weinberger wrote:
> >> UML's block device driver does not support write barriers,
> >> to support this this patch adds REQ_FLUSH suppport.
> >> Every time the block layer sends a REQ_FLUSH we fsync() now
> >> our backing file to guarantee data consistency.
> >
> > This fixes the sync problem I saw before.  So:
> 
> What's the impact on your performance figures?

It just moves the sync into the main process, thus making the
"write-no-upload" test time *increase* to what we expected (the same
as KVM).

This doesn't address the main performance issue, which appears to lie
in the serial port emulation.

Rich.

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