From: Ric Wheeler <rwheeler@redhat.com>
Subject: Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:
 document conditions when reliable operation is possible)
Date: Thu, 03 Sep 2009 09:50:43 -0400
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Cc: Christoph Hellwig <hch@infradead.org>, Mark Lord <lkml@rtr.ca>,
	Michael Tokarev <mjt@tls.msk.ru>, david@lang.hm,
	Pavel Machek <pavel@ucw.cz>, Theodore Tso <tytso@mit.edu>,
	NeilBrown <neilb@suse.de>, Rob Landley <rob@landley.net>,
	Florian Weimer <fweimer@bfk.de>,
	Goswin von Brederlow <goswin-v-b@web.de>,
	kernel list <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@osdl.org>, mtk.manpages@gmail.com,
	rdunlap@xenotime.net, linux-doc@vger.kernel.org,
	linux-ext4@vger.kernel.org, corbet@lwn.net
To: Krzysztof Halasa <khc@pm.waw.pl>
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On 09/03/2009 09:34 AM, Krzysztof Halasa wrote:
> Ric Wheeler<rwheeler@redhat.com>  writes:
>
>>>> Just to add some support to this, all of the external RAID arrays that
>>>> I know of normally run with write cache disabled on the component
>>>> drives.
>>>
>>> Do they use "off the shelf" SATA (or PATA) disks, and if so, which ones?
>>
>> Which drives various vendors ships changes with specific products.
>> Usually, they ship drives that have carefully vetted firmware, etc.
>> but they are close to the same drives you buy on the open market.
>
> But they aren't the same, are they? If they are not, the fact they can
> run well with the write-through cache doesn't mean the off-the-shelf
> ones can do as well.

Storage vendors have a wide range of options, but what you get today is a 
collection of s-ata (not much any more), sas or fc.

Some times they will have different firmware, other times it is the same.


>
> Are they SATA (or PATA) at all? SCSI etc. are usually different
> animals, though there are SCSI and SATA models which differ only in
> electronics.
>
> Do you have battery-backed write-back RAID cache (which acknowledges
> flushes before the data is written out to disks)? PC can't do that.

We (red hat) have all kinds of different raid boxes...

ric