linux-kernel - Re: limits on raid

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Date:	Sat, 16 Jun 2007 10:08:57 -0400
From:	Wakko Warner <wakko@...mx.eu.org>
To:	Neil Brown <neilb@...e.de>
Cc:	david@...g.hm, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org
Subject: Re: limits on raid

Neil Brown wrote:
> On Friday June 15, wakko@...mx.eu.org wrote:
>  
> >                                                   As I understand the way
> > raid works, when you write a block to the array, it will have to read all
> > the other blocks in the stripe and recalculate the parity and write it out.
> 
> Your understanding is incomplete.
> 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.

I see, I didn't know that the MD's raid5 did this.

> And why is it such a big deal anyway?  The initial resync doesn't stop
> you from using the array.  I guess if you wanted to put an array into
> production instantly and couldn't afford any slowdown due to resync,
> then you might want to skip the initial resync.... but is that really
> likely?

When I've had an unclean shutdown on one of my systems (10x 50gb raid5) it's
always slowed the system down when booting up.  Quite significantly I must
say.  I wait until I can login and change the rebuild max speed to slow it
down while I'm using it.   But that is another thing.

Thanks for the clarification on raid5.

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