Microcontrollers/ASICs v. General Purpose Microprocessors -- WAS: Opteron Vs. Athlon X2

[Eugen Leitl]

On Fri, Dec 09, 2005 at 01:09:58AM -0800, Bryan J. Smith wrote:

> Why use a layer-3 switch?

I use one for VLAN isolation for each host, and because 
it has native intelligence so it can run diagnostics 
and be remotely administered. For Fast Ethernet, Layer 3
switches are not really expensive.
 
> I mean, a layer-2 switch with a router on one port would be
> cheaper?

For storage (AoE) a dumb switch would be more than sufficient.
 
> Why put in a HBA for FC-AL or iSCSI?  I mean, with a fast
> enough system, GbE is only about 100MBps, so why can't a
> system handle it?

You could certainly roll a pretty performant AoE or a Lustre or 
PVFS system over a rack of 4-disk servers, especially with a 
10 GBit Ethernet uplink.
 
> After all, TCP segmentation is only like addresses of windows
> into packets of a larger blocks -- much like sectors are to
> the windows of partitions on a storage array?

Why would one want to use TCP/IP on a LAN storage network when
naked Ethernet frames would do nicely?
 
> Especially when these powerful, costly layer-3 switches only
> have a 200, 300 ... maybe 400MHz MIPS or ARM/XScale at their
> cores?

They're only there to control the ASIC and to do remote
administration (ssh, web, Java UI). 
 
> Heck, why aren't I using my server as a layer-3/4 router?! 
> Throw in some 4-port GbE cards and I can do it better and
> cheaper than a layer-3 switch!  So why not?

But for the failure rate (requiring failover) and power requirements
there are no reasons I'm aware of. Modern TCP/IP stacks can achieve
wire speed on 10 GBit Ethernet -- assuming you don't need the CPU 
for much else.

> P.S.  There is a reason why 3Ware calls the Escalade
> 7000/8000 a "Storage Switch" -- it's design is what you can
> expect in a layer-3 router.  A microcontroller core with ASIC
> peripherals and SRAM cache (just like network ASICs around a
> microcontroller core) designed for one thing, queue, move and
> replicate data.  Now I would _not_ use a 7000/8000 series
> with only 1-4MB of SRAM for RAID-5 anymore than I would/could
> use a layer-3 Ethernet switch that only has 1-4MB of SRAM for
> NAT/PAT and network filtering duties.  But there are more
> advanced cards, just like their are more advanced networking
> equipment -- all often powered by little more than one
> RISC/microcontroller core of a few hundred MHz.

Most reasons why software RAID works so well is because cheap
CPU and large memory are commodity, and a spare is a lot cheaper
and easier to find.

-- 
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
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