Software vs Hardware RAID

[Tom Metro]

Matthew Gillen wrote:
> Software RAID might get a bad rap since it's more often used
> in conjunction with cheap hardware...

I think the bad rap occurred due to early implementation that weren't as 
reliable as modern day implementations of software RAID.

There's a cross-over point on the performance curve where above it you 
go hardware RAID, and below it you go software RAID. Modern CPUs and 
software RAID implementations have been steadily pushing this cross over 
point higher.

Years ago the extra work of calculating parity (and other RAID related 
calculations) was enough of a burden for the main CPU, that it didn't 
take much disk I/O to make it beneficial to offload that to a dedicated 
co-processor (hardware RAID). CPUs, now commonly multicore, can easily 
do these calculations while hardly putting any load on the CPU.

This is why modern software RAID implementations can sometimes out 
perform low-end hardware RAID solutions, and fake RAID, which is just a 
proprietary software RAID implemented by some controller card vendor.

So unless you are dealing with a high performance system, the biggest 
practical difference between the approaches are compatibility issues. 
Once your RAID set is configured, if you use hardware RAID, you have to 
stick with the same controller type (vendor). If you use software RAID, 
you can use almost any controller, but you have to stick with the same 
software RAID implementation. For most people these limitations are 
irrelevant, unless they are dual-booting a system or moving their RAID 
array among systems.

I see Kristian Hermansen's post says that there is a fake RAID 
implementation that offers compatibility across multiple operating 
systems. I guess that would be the main advantage to fake RAID, if you 
needed that.

(BTW, most low-cost RAID controllers on the market are of the fake RAID 
type. Typically you can ignore the proprietary drivers and use them as 
ordinary controllers with software RAID under Linux.)

  -Tom

-- 
Tom Metro
Venture Logic, Newton, MA, USA
"Enterprise solutions through open source."
Professional Profile: http://tmetro.venturelogic.com/

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