linux-kernel - Re: MD raid and different elevators (disk i/o schedulers) (fwd)

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Message-ID: <alpine.DEB.1.10.1007301434080.9875@uplift.swm.pp.se>
Date:	Fri, 30 Jul 2010 14:40:46 +0200 (CEST)
From:	Mikael Abrahamsson <swmike@....pp.se>
To:	linux-kernel@...r.kernel.org
Subject: Re: MD raid and different  elevators  (disk i/o schedulers) (fwd)

Hi, this might be more appropriate for lkml (or is there another place?) 
because people with knowledge of how these layers interact might be here 
and not on linux-raid-ml ?

If block cache is done on all levels and readahead is done on all levels, 
then quite a lot of redundant block information is going to exist in 
memory for all these layers? I can understand that it might make sense to 
keep block cache for the fs and perhaps for the drive layer, but 
md->dm(crypto)->lvm layers this might make less sense?

What about default readahead for these devices? Doing readahead on dm 
device might be bad in some situations, perhaps good in others?

---------- Forwarded message ----------
Date: Thu, 29 Jul 2010 12:53:35 +0200 (CEST)
From: Mikael Abrahamsson <swmike@....pp.se>
To: Fabio Muzzi <liste@...gan.org>
Cc: linux-raid@...r.kernel.org
Subject: Re: MD raid and different  elevators  (disk i/o schedulers)

On Thu, 29 Jul 2010, Fabio Muzzi wrote:

> Is this true? Are there compatibility issues using different i/o schedulers 
> with software raid?

I'd actually like to raise this one level further:

In the case of (drives)->md->dm(crypto)->lvm->fs, how do the schedulers, 
readahead settings, blocksizes, barriers etc interact thru all these layers? Is 
block caching done on all layers? Is readahead done on all layers?

-- 
Mikael Abrahamsson    email: swmike@....pp.se
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