linux-kernel - Re: Help with implementing some form of barriers in 3.0 kernels.

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Message-Id: <4E6892B4020000780005536B@nat28.tlf.novell.com>
Date:	Thu, 08 Sep 2011 09:02:28 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	<hch@...radead.org>
Cc:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"Vivek Goyal" <vgoyal@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: Help with implementing some form of barriers in 3.0
	 kernels.

>>> On 07.09.11 at 22:16, Christoph Hellwig <hch@...radead.org> wrote:
> [Hmm, for some reason I never manage to receive Konrads mails directly,
>  but only get the replies, or copies via the list]
> 
> On Wed, Sep 07, 2011 at 02:17:40PM -0400, Vivek Goyal wrote:
>> On Wed, Sep 07, 2011 at 01:48:32PM -0400, Konrad Rzeszutek Wilk wrote:
>> > Hey Christoph,
>> > 
>> > I was wondering what you think is the proper way of implementing a
>> > backend to support the 'barrier' type requests? We have this issue were
>> > there are 2.6.36 type guests that still use barriers and we would like
>> > to support them properly. But in 3.0 there are no barriers - hence
>> > the question whether WRITE_fLUSH_FUA would be equal to WRITE_BARRIER?
>> 
>> I think WRITE_FLUSH_FUA is not same as WRITE_BARRIER. Because it does
>> not ensure request ordering. A request rq2 which is issued after rq1 (with
>> WRITE_flush_FUA), can still finish before rq1. In the past WRITE_BARRIER
>> would not allow that.
>> 
>> So AFAIK, WRITE_flush_fua is not WRITE_BARRIER.
> 
> Indeed.  And while most guests won't care some will.  E.g. reiserfs
> which is the standard filesystem in most SuSE guests, which happen to
> be fairly popular with Xen.
> 
> I'd suggest you look at the pre-2.6.36 barrier implementation and see
> if you can move that into xen-blkfront.

That would need to be done in the backend, as the expectation cannot
be to patch old frontends to deal with new backends.

Jan

> For the qemu side doing this is a bit easier as you'll just have to wait
> for all pending aio requests to complete.  The current qemu xen disk
> code gets thus horribly wrong, though.



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