building a RAID system

Trent Piepho xyzzy at speakeasy.org
Thu Oct 9 16:35:04 EDT 2003


On Thu, 9 Oct 2003, Daniel Fernandez wrote:
> On Thu, 2003-10-09 at 01:39, Mark Hahn wrote:
> > > I would like to know some advice about what kind of technology apply
> > > into a RAID file server ( through NFS ) . We started choosing hardware
> > > RAID to reduce cpu usage.
> > 
> > that's unfortunate, since the main way HW raid saves CPU usage is 
> > by running slower ;)
> > 
> I cannot get the point here, the dedicated processor should take all
> transfer commands and offload the CPU why it would run slower ? In some

Easy, said dedicated processor and memory is quite a bit slower than the main
CPU and memory.  If you look at thoughput in MB/sec, the latest linux software
RAID is usually much faster than hardware raid implimentations.

Usually CPU usage is (stupidly) reported as just a % used during a benchmark. 
If you transfer fewer megabytes in second, obviously the number of CPU cycles
used in that second go down as well.  If CPU usage is correctly reported in
units of % per MB/sec, then you get a real measure of hardware efficiency.

> needed, and 4 channels at least... 4 HDs in 2 master/slave channels
> reduces drastically performance
> ¿ any controller recommended ?

It seems that most good 4-12 channel (NOT drive, channel!) IDE cards ARE
hardware raid controllers.  Lots of people use the 3ware RAID cards in JBOD
mode with software raid, because their isn't a cheaper non-hardware raid card
comparable to something like the 3ware 7508-8 or 7508-12.  I know about
cheaper 2 and 4 channel non-raid cards, but they're 32/33 PCI and not
comparable to the 3ware.


> > concern for PCI congestion is a much more serious issue.
> > 
> We're limited at 32 bit PCI, we cannot get around this unless spend on a
> highly priced PCI 64 mainboard.

AMD 760MPX and Intel E7501 motherboards have high speed 64/66 PCI and PCI-X
for the E7501.  They're not that expensive really.  An additional $100-$200 at
most over a single PCI 32/33 motherboard.

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