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Re: XFS and write barriers.

To: David Chinner <dgc@xxxxxxx>
Subject: Re: XFS and write barriers.
From: Neil Brown <neilb@xxxxxxx>
Date: Mon, 26 Mar 2007 10:01:01 +1000
Cc: Neil Brown <neilb@xxxxxxx>, Timothy Shimmin <tes@xxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: message from David Chinner on Sunday March 25
References: <17923.11463.459927.628762@xxxxxxxxxxxxxx> <1755676AA526FF7790546385@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> <17923.35118.139991.252734@xxxxxxxxxxxxxx> <20070325031927.GG32602149@xxxxxxxxxxxxxxxxx>
Sender: xfs-bounce@xxxxxxxxxxx
On Sunday March 25, dgc@xxxxxxx wrote:
> On Fri, Mar 23, 2007 at 07:00:46PM +1100, Neil Brown wrote:
> > 
> > Why no barriers on an external log device??? Not important, just
> > curious.
> 
> because we need to synchronize across 2 devices, not one, so issuing
> barriers on an external log device does nothing to order the metadata
> written to the other device...

Right, of course.  Just like over a raid0.

So you must have code to wait for all writes to the main device before
writing the commit block on the journal.   How hard is it to fall-back
to that if the barrier fails?

NeilBrown


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