linux-kernel - Re: [BUG] scsi: hpsa: how to destroy your files

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Date:	Fri, 2 Sep 2011 10:03:21 -0500
From:	scameron@...rdog.cce.hp.com
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jon Mason <mason@...i.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	stephenmcameron@...il.com, thenzl@...hat.com,
	akpm@...ux-foundation.org, mikem@...rdog.cce.hp.com,
	linux-pci@...r.kernel.org, Roland Dreier <roland@...estorage.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [BUG] scsi: hpsa: how to destroy your files

On Fri, Sep 02, 2011 at 12:08:53PM +0200, Eric Dumazet wrote:
> Le vendredi 02 septembre 2011 à 11:39 +0200, Eric Dumazet a écrit :
> > Le jeudi 01 septembre 2011 à 16:50 -0500, Jon Mason a écrit :
> > 
> > > I believe modifying the MRRS values is what is causing the issues.
> > > Can you try the attached patch and verify that it also resolves the
> > > issue?
> > 
> > I tested this patch and can confirm this solves the corruption problem.
> > 
> > But my disk is _much_ slower than before
> > 
> > # hdparm -t /dev/sda1
> > 
> > Before :
> > 
> >  Timing buffered disk reads: 254 MB in  3.02 seconds =  84.16 MB/sec
> > 
> > After :
> > 
> >  Timing buffered disk reads: 120 MB in  3.04 seconds =  39.42 MB/sec
> 
> Hmm, this speed regression is probably old : the 84MB/s was with the
> standard debian 6.0.2 kernel (2.6.32-5-amd64)
> 

This regression might be due to these two patches:

	d0be5ec8693944c2e2fc0de70fda9dbc1b93bd7d
	[SCSI] hpsa: do readl after writel in main i/o path to ensure commands don't get lost.

	Apparently we've been doin it rong for a decade, but only lately do we
	run into problems.
and
	fec62c368b9c8b05d5124ca6c3b8336b537f26f3
	[SCSI] hpsa: do not attempt to read from a write-only register

	Most smartarrays tolerate it, but a few new ones don't.
	Without this change some newer Smart Arrays will lock up
	and i/o will grind to a halt.

with the second patch being a correction to the first.

It seems like the readl after the writel should not be needed,
and wasn't needed for a very long time, but there is some very
hard to trigger and not yet well understood problem in which very
occasionally a command would get lost and the driver thinks a 
command is out, but controller firmware thinks all commands are
completed -- a circumstance which tends to make things grind to
a halt.

Those two patches avoid that problem.

-- steve

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