linux-kernel - Re: solid state drive access and context switching

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Message-ID: <6934efce0712040954v74cf0b4bk19b49988bc828233@mail.gmail.com>
Date:	Tue, 4 Dec 2007 09:54:54 -0800
From:	"Jared Hulbert" <jaredeh@...il.com>
To:	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Cc:	"Chris Friesen" <cfriesen@...tel.com>, linux-kernel@...r.kernel.org
Subject: Re: solid state drive access and context switching

> > Has anyone played with this concept?
>
> For things like SATA based devices they aren't that fast yet.

What is fast enough?

As I understand the basic memory technology, the hard limit is in the
100's of microseconds range for latency.  SATA adds something to that.
 I'd be surprised to see latencies on SATA SSD's as measured at the OS
level to get below 1 millisecond.

What happens we start placing NAND technology in lower latency, higher
bandwidth buses?  I'm guessing we'll get down to that 100's of
microseconds level and an order of magnitude higher bandwidth than
SATA.  Is that fast enough to warrant this more synchronous IO?

Magnetic drives have latencies ~10 milliseconds, current SSD's are an
order of magnitude better (~1 millisecond), new interfaces and
refinements could theoretically get us down one more (~100
microsecond).  I'm guessing the current block driver subsystem would
negate a lot of that latency gain.  Am I wrong?

BTW - This trend toward faster, lower latency busses is marching
forward.  2 examples; the ioDrive from Fusion IO, Micron's RAM-module
like SSD concept.
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