Date: Sat, 15 Mar 2008 21:03:08 +0000
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Daniel Phillips <phillips@phunq.net>
Cc: Willy Tarreau <w@1wt.eu>, David Newall <davidn@davidnewall.com>,
       linux-kernel@vger.kernel.org
Subject: Re: [ANNOUNCE] Ramback: faster than a speeding bullet
Message-ID: <20080315210308.72824eb3@the-village.bc.nu>
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	<200803151417.13899.phillips@phunq.net>
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> > RAID controllers do not have half a terabyte of RAM.
> 
> And?  Either you have battery backed ram with critical data in it or
> you do not.  Exactly how much makes little difference to the question.

It makes a lot of difference, and in addition raid controllers (good
ones) respect barrier ordering in their RAM cache so they'll take tags or
similar interfaces and honour them.

> That is why I keep recommending that a ramback setup be replicated or
> mirrored, which people in this thread keep glossing over.  When
> replicated or mirrored, you still get the microsecond-level transaction
> times, and you get the safety too.

Either you keep a mirror in sync and get normal data rates or you keep
the mirror out of sync and then you need to sort your writeback process
out to preserve ordering.

If you want ramback to be taken seriously then that is the interesting
problem to solve and clearly has multiple solutions if you would start to
take an objective look at your work.

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