Re: hot-add unformatted drive to RAID array automagically
On Saturday 19 February 2005 00:30, Wouter Verhelst <wouter@debian.org> wrote:
> > Not if the dedicated processor has the performance of a 486 and the
> > general purpose processor is a P4 or Athlon.
>
> Sure, but I'd think someone who sells hardware RAID controllers today
> with a processor that has the performance of a 486, they need to
> seriously look into their business plans.
Traditionally the controllers on the hardware RAID devices have been severely
under-powered.
> I haven't done any benchmarks, and no longer have access to the hardware
> I was talking about (previous job). Back then, I regularly had to set up
> RAID-based systems where the increased availability of the system was
> far more important than the speed (systems were being used for a
In which case a slow hardware RAID system should do well.
> Theoretically, having a dedicated processer also means the main
> processor is free to do other things (as long as it doesn't require disk
> access). If the controller is up to snuff, it should thus be faster.
On loading raid5.ko a P2-400 machine reports that it can do checksumming at a
rate of 1064MB/s, a P-M 1.7G reports 1588MB/s, and a Celeron 1.8G reports
2260MB/s. Those speeds are all so much faster than the IO capacity of the
machines in question that there's no chance of checksum operations taking
more than 10% CPU time.
The main performance benefit that you can hope to get from hardware RAID in
this regard is reducing the volume of data that goes through system buses. A
system with 33MHz 32bit PCI can only do about 200MB/s - which is four
moderately fast disks doing sequential IO. With hardware RAID on such as
system you can have five disks in a RAID array going at full speed while
software RAID has an IO bottleneck at four disks.
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