linux-kernel - Re: limits on raid

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Message-ID: <Pine.LNX.4.64.0706160037220.7557@asgard.lang.hm>
Date:	Sat, 16 Jun 2007 00:48:57 -0700 (PDT)
From:	david@...g.hm
To:	Neil Brown <neilb@...e.de>
cc:	Wakko Warner <wakko@...mx.eu.org>, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org
Subject: Re: limits on raid

On Sat, 16 Jun 2007, Neil Brown wrote:

> It would be possible to have a 'this is not initialised' flag on the
> array, and if that is not set, always do a reconstruct-write rather
> than a read-modify-write.  But the first time you have an unclean
> shutdown you are going to resync all the parity anyway (unless you
> have a bitmap....) so you may as well resync at the start.
>
> 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?

in my case it takes 2+ days to resync the array before I can do any 
performance testing with it. for some reason it's only doing the rebuild 
at ~5M/sec (even though I've increased the min and max rebuild speeds and 
a dd to the array seems to be ~44M/sec, even during the rebuild)

I want to test several configurations, from a 45 disk raid6 to a 45 disk 
raid0. at 2-3 days per test (or longer, depending on the tests) this 
becomes a very slow process.

also, when a rebuild is slow enough (and has enough of a performance 
impact) it's not uncommon to want to operate in degraded mode just long 
enought oget to a maintinance window and then recreate the array and 
reload from backup.

David Lang
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