linux-kernel - Re: [PATCH] md: Add support for data integrity to MD

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Message-ID: <yq1ljxmmf46.fsf@sermon.lab.mkp.net>
Date:	Sat, 20 Sep 2008 22:01:29 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	"Dan Williams" <dan.j.williams@...il.com>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	jens.axboe@...cle.com, neilb@...e.de, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org
Subject: Re: [PATCH] md: Add support for data integrity to MD

>>>>> "Dan" == Dan Williams <dan.j.williams@...il.com> writes:

>> + /* Data integrity passthrough not supported on RAID 4, 5 and 6 */
>> + if (pers->level >= 4 && pers->level <= 6) + return 0;

Dan> Do you have a reference to the issues in supporting this?

Not really.

The protection information is an extra 8-bytes of stuff per sector.
But it comes down in a different buffer.  To do R-M-W effectively
we'll have to maintain a separate protection information stripe cache.
And we'll have to generate DIF checksums and reference tags for the
parity sectors going out.

Quite frankly it's a lot of ick and I haven't completely wrapped my
head around it yet.

-- 
Martin K. Petersen	Oracle Linux Engineering

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