IBM serveraid and linux

Henry Spencer henry-lqW1N6Cllo0sV2N9l4h3zg at public.gmane.org
Thu Jan 6 06:09:13 UTC 2005


On Thu, 6 Jan 2005, Dave Stubbs wrote:
> ...Wouldn't it be faster?  Unless (I guess) the modern CPUs 
> are just so powerful they can run the calc faster than the dedicated 
> copro on the RAID card.

Remember that modern CPUs have a cycle time of a fraction of a nanosecond.
That dedicated coprocessor probably doesn't have anywhere near that clock
speed, as witness the fact that it doesn't have its own howling cooling
fan.  Unless it's a custom design, which it probably isn't, it can't do
that calculation anywhere *NEAR* as fast as the main CPU can. 

The data takes longer to get through the *wires* than the main CPU needs
to run parity on it.  (An electric signal travels only a few centimeters
along a wire in one main-CPU clock cycle!)

This has always been the problem with such I/O-card coprocessors:  unless
they have a radically different architecture than the main CPU, so much
better suited to the problem that they inherently handle it much more
efficiently, they will always be much slower than the main CPU, so they're
usually pretty useless.  By their nature, they can't command anywhere near
the level of resources lavished on making the main CPU fast.  And the
mind-boggling acceleration of CPU speeds in the last decade has made this
problem even worse. 

                                                          Henry Spencer
                                                       henry-lqW1N6Cllo0sV2N9l4h3zg at public.gmane.org

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