[Pc_Support] Re: Intel ICH7R and nVidia MCP-04 at RAID-5 = 15MBps (yes, it's FRAID)

[Bryan J. Smith]

On Mon, 2005-09-05 at 13:15 -0400, Damien McKenna wrote:
> Also note that many nVidia nForce 4 motherboards have an SIL3114 chip 
> that does RAID-5... in software.  Even SIL's online documentation 
> doesn't mention RAID-5 support in that chip (you need the SIL3124 for 
> that), its a software add-on.

A lot of vendors are starting to move into the "software RAID options as
a profit model."  Broadcom and others are selling software RAID support
such as dynamic resizing and other things as add-ons for $$$.

I have no problem with that as long as the solution does at least RAID-1
(or RAID-10 aka RAID-1e) duplication _in_hardware_, so I'm not turning
my CPU into an I/O processor, and reducing my performance.  And that's
just for RAID-1 or 10.

For RAID-5, I expect XORs to be done locally in local hardware.  If it's
being done at the CPU, I'm better off using the OS' RAID functionality
like NT5+'s LDM, Linux's MD, etc... -- which still sucks compared to an
on-board XOR engine.  Remember, it's not the simple XOR operation that
kills you.  It's all the extra dataflow in your system.

It's much, much, _much_ cheaper for these companies to sell you a
software RAID-5 solution than to make a hardware one -- literally the
difference between a $5 card and a $150 card.  The RAID license is
typically a 1-time licensing deal, whereas making a true, intelligent
hardware RAID solution is a massive, recurring cost.

CPU utilization is _not_ a good indicator of the performance hit.  RAID
benchmarks are best showing "real world" file/application server
performance as the RAID is running.  That's where the added I/O of
software RAID cuts massively into the services performance -- especially
for writes (and God help you during rebuilds ;-).  Software RAID _only_
works well when the system is doing nothing else -- i.e., the system is
a storage device, not a file/application/etc... server as well.

When spending $3,000 on a server, I have a real problem when people want
to save $300 -- $300 that would give them far greater performance and
far fewer headaches.  I have yet to meet someone who has deployed 3Ware
solutions (_correctly_**) that has gone back to FRAID or even software
RAID.

-- Bryan

**NOTE:  I have seen plenty of people buy 3Ware cards and then use
software RAID, only to complain about 3Ware.  They buy the 3Ware cards
for hot-swap support, which doesn't always work correctly with software
RAID.  I find it incomprehensible to buy a 3Ware card and not use its
hardware RAID functions.  9 times out of 10, these people have not even
attempted it, but believed the over-marketing non-sense of software RAID
bigots who say hardware RAID is slower -- typically based on their prior
use of i960 hardware RAID solutions that were not viable 10 years ago,
much less 5 years ago when StrongARM and ASICs started showing up.


-- 
Bryan J. Smith     b.j.smith at ieee.org     http://thebs413.blogspot.com
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