linux-kernel - Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

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Date:	Wed, 30 May 2007 10:55:13 +0200
From:	"Stefan Bader" <Stefan.Bader@...ibm.com>
To:	"David Chinner" <dgc@....com>
Cc:	david@...g.hm, "Phillip Susi" <psusi@....rr.com>,
	"Neil Brown" <neilb@...e.de>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com,
	linux-raid@...r.kernel.org, "Jens Axboe" <jens.axboe@...cle.com>,
	"Andreas Dilger" <adilger@...sterfs.com>,
	"Tejun Heo" <htejun@...il.com>
Subject: Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

> The order that these are expected by the filesystem to hit stable
> storage are:
>
> 1. block 4 and 10 on stable storage in any order
> 2. barrier block X on stable storage
> 3. block 5 and 20 on stable storage in any order
>
> The point I'm trying to make is that in XFS,  block 5 and 20 cannot
> be allowed to hit the disk before the barrier block because they
> have strict order dependency on block X being stable before them,
> just like block X has strict order dependency that block 4 and 10
> must be stable before we start the barrier block write.
>

That would be the exactly how I understand Documentation/block/barrier.txt:

"In other words, I/O barrier requests have the following two properties.
1. Request ordering
...
2. Forced flushing to physical medium"

"So, I/O barriers need to guarantee that requests actually get written
to non-volatile medium in order."

Stefan
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