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Re: RAID5/6 writes

To: Peter Cordes <peter@xxxxxxxxx>
Subject: Re: RAID5/6 writes
From: Andi Kleen <andi@xxxxxxxxxxxxxx>
Date: Wed, 1 Oct 2008 22:44:50 +0200
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20081001201331.GL32037@xxxxxxxxx>
References: <20081001175237.GJ32037@xxxxxxxxx> <87k5csp0pe.fsf@xxxxxxxxxxxxxxxxx> <20081001201331.GL32037@xxxxxxxxx>
User-agent: Mutt/1.4.2.1i
On Wed, Oct 01, 2008 at 05:13:31PM -0300, Peter Cordes wrote:
> On Wed, Oct 01, 2008 at 09:36:13PM +0200, Andi Kleen wrote:
> > Peter Cordes <peter@xxxxxxxxx> writes:
> > >
> > >  XFS knows (or should have been told by the admin with mkfs!) what the
> > > stripe geometry is: block size and stripe width.  So it could apply
> > > this optimization only if it would make a write cover more whole
> > > blocks or whole stripes.
> > 
> > It's a nice idea, but I don't think XFS knows the actual RAID level,
> > only the stripes. And for 0/1 it wouldn't be a good idea.
> 
>  Yeah, this would have to be a mount option, like stripewrite=1.
> There are already a few other essential mount options people need to
> learn about for big RAIDs, e.g. inode64.

The other problem I can think of is that determing if something is 
free data might need more read IO if the free extent tree is not completely
cached.

-Andi

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