linux-kernel - Re: limits on raid

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Message-ID: <a56dfcf00706152140s36d01ff6sb9a6bb716b8afe65@mail.gmail.com>
Date:	Sat, 16 Jun 2007 00:40:17 -0400
From:	"Dan Merillat" <dan.merillat@...il.com>
To:	"Neil Brown" <neilb@...e.de>
Cc:	"Wakko Warner" <wakko@...mx.eu.org>, david@...g.hm,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org
Subject: Re: limits on raid

> For raid5 on an array with more than 3 drive, if you attempt to write
> a single block, it will:
>
>  - read the current value of the block, and the parity block.
>  - "subtract" the old value of the block from the parity, and "add"
>    the new value.
>  - write out the new data and the new parity.
>
> If the parity was wrong before, it will still be wrong.  If you then
> lose a drive, you lose your data.

Wow, that really needs to be put somewhere in 120 point red blinking
text.  A lot of us are used to uninitialized disks calculating the
parity-on-first-write, but if linux MD is forgoeing that
'dangerous-no-resync' sounds really REALLY bad.  How about at least a
'Warning: unlike other systems this WILL cause corruption if you
forego reconstruction' on mkraid?
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