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Message-ID: <20051230161805.64631.qmail@web34110.mail.mud.yahoo.com>
Date: Fri, 30 Dec 2005 08:18:05 -0800 (PST)
From: Kenny Simpson <theonetruekenny@yahoo.com>
Subject: Re: RAID controller safety
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: linux kernel <linux-kernel@vger.kernel.org>
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> >   Specificly, I am looking at the Adaptec RAID controllers and their i2o drivers.  I am told
> the
> > kernel's i2o driver lacks a strong guarantee on fsync, and so far am unable to determine if
> the
> > dpt_i2o driver also falls short in this reguard.
> 
> Only dpt can tell you what their firmware actually does.

Yeah, I wasn't so much interrested in the firmware just yet, just interrested if the device driver
(dpt_i2o) gave it a fighting chance of doing the right thing.

> The i2o core drivers use the following rules

> i2o_block by default assumes the card is caching. It adopts write
> through mode if the controller has no battery, write back if it shows
> battery. This can be configured differently via ioctls including the
> ability to tune write through of large I/O's (to avoid cache thrashing),
> and to do write back with no battery backup for performance in cases
> where losing the data on a crash doesn't matter (eg swap)

That's what I read in the comments too, but looking at the code I only ever see it set to
write-back.  I verified this with blktool - our controllers have no battery, and blktool showed
the i2o-wcache state as write-back.

However, I was also told that the i2o_block driver lacks barrier support, so even in the
write-back case, the controller won't be told to flush/sync.

I was sent a patch against 2.6.10 that implements barrier support in i2o_block, but the code base
has shifted too much for me to make it apply.

-Kenny


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