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

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Date:	Mon, 17 Mar 2008 18:30:01 +0100
From:	Willy Tarreau <w@....eu>
To:	david@...g.hm
Cc:	Daniel Phillips <phillips@...nq.net>,
	David Newall <davidn@...idnewall.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: [ANNOUNCE] Ramback: faster than a speeding bullet

On Mon, Mar 17, 2008 at 10:23:10AM -0700, david@...g.hm wrote:
> 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never out of date.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)

You can do that with 3 10GE NICS, though in practise that's not easy. 

Willy

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