linux-ext4 - Re: raid is dangerous but that's secret (was Re: [patch] ext2/3: document conditions when reliable operation is possible)

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Message-ID: <alpine.DEB.2.00.0909030732560.6822@asgard.lang.hm>
Date:	Thu, 3 Sep 2009 07:35:14 -0700 (PDT)
From:	david@...g.hm
To:	Krzysztof Halasa <khc@...waw.pl>
cc:	Ric Wheeler <rwheeler@...hat.com>,
	Christoph Hellwig <hch@...radead.org>, Mark Lord <lkml@....ca>,
	Michael Tokarev <mjt@....msk.ru>, Pavel Machek <pavel@....cz>,
	Theodore Tso <tytso@....edu>, NeilBrown <neilb@...e.de>,
	Rob Landley <rob@...dley.net>, Florian Weimer <fweimer@....de>,
	Goswin von Brederlow <goswin-v-b@....de>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org, corbet@....net
Subject: Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:
 document conditions when reliable operation is possible)

On Thu, 3 Sep 2009, Krzysztof Halasa wrote:

> Ric Wheeler <rwheeler@...hat.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.

frequently they are exactly the same drives, with exactly the same 
firmware.

you disable the write caches on the drives themselves, but you add a large 
write cache (with battery backup) in the raid card/chassis

> 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.

it depends on what raid array you use, some use SATA, some use SAS/SCSI

David Lang
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