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

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Date:	Mon, 17 Mar 2008 10:23:10 -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, david@...g.hm wrote:

> 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 never out of date.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.

by the way, the only way to get this much bandwideth between two machines 
is to directly connect PCI-e/16 card slots togeather. this is definantly 
not commodity hardware anymore (if it's even possible, PCI-e has some very 
short distance limitations)

David Lang
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