linux-kernel - Re: [ANNOUNCE] Ramback: faster than a speeding bullet

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Message-ID: <alpine.DEB.1.00.0803170740440.14887@asgard.lang.hm>
Date:	Mon, 17 Mar 2008 07:42:59 -0700 (PDT)
From:	david@...g.hm
To:	Daniel Phillips <phillips@...nq.net>
cc:	David Newall <davidn@...idnewall.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, Willy Tarreau <w@....eu>,
	linux-kernel@...r.kernel.org
Subject: Re: [ANNOUNCE] Ramback: faster than a speeding bullet

On Mon, 17 Mar 2008, Daniel Phillips wrote:

> On Sunday 16 March 2008 23:49, david@...g.hm wrote:
>>> Mirroring on the other hand, makes a realtime copy of a volume, that is
>>> never out of date.
>>
>> so just mirror to a local disk array then.
>
> Great idea.  Except that the disk array has millisecond level latency,
> when what we trying to achieve is microsecond level latency.
>
>> a local disk array has more write bandwidth than a network connection to a
>> remote machine, so if you can mirror to a remote machine you can mirror to
>> a local disk array.
>
> So you could potentially connect to a _huge_ disk array and write deltas
> to it.  The disk array would have to support roughly 3 Gbytes/second of
> write bandwidth to keep up with the Violin ramdisk.  Doable, but you are
> now in the serious heavy iron zone.

your network will do less then 1 Gbit/sec, so to mirror in real-time (what 
you claim is trivial) you would need at least 24 network connections in 
parallel. that's a LOT harder to setup then a high performance disk array.

David Lang
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