HW RAID vs SW RAID

HW RAID vs SW RAID

Post by thomas madse » Thu, 18 May 2000 04:00:00



Hi all,

I've recently experienced a double diskcrash on a f50 (running aix 4.3.3)
with mirrorred disks. The two disks which crashed were mirrors of eachother.
The mirror was SW controlled.

A colleague later told me that he had experienced exactly the same thing.

It is possible that the SW controlled mirrors are that unstable? Is there
any way to improve a SW controlled mirror?

- thomas

 
 
 

1. hw vs sw RAID-0

I would like to know whether hardware or software RAID is better for my
particular application.  I have two Seagate Cheetah 2.2 Gb SCSI disks in
a RAID-0 array controlled by a DPT PM2044UW controller to which is
attached a DPT RC4040 memory adapter with 8 Mb of RAM.  This array is
used as the scratch space (as 1 large 4.4 Gb partition) for molecular
modeling calculations.  The OS and molecular modeling software are run
from a separate IDE hard drive.  The scratch files include one large
binary file which is usually 200 Mb in size but sometimes larger than 2
Gb.  During one particularly long calculation I noticed that the RAID
array was being accessed every 8 seconds.  In general, the I/O to the
array is regular, not random, although the frequency of access can vary
depending on the size of the calculation.  For a particular
calculation, I think the size of each I/O transfer is probably
constant, but the size should differ quite a bit from calculation to
calculation (the I/O data consists of the values to many integrals; the
larger the molecule, the more integrals and the larger the I/O sizes).
I am currently using hardware RAID via the DPT controller, with a stripe
size of 128K (the default).  I am using RedHat Linux 5.2 and the 2.2.9
kernel configured for SMP with two PII-450 cpus.

I read in one of the RAID HOWTOs that software RAID can in fact be
faster than hardware RAID, but no details were given on how to tell.  As
I am not an expert, I don't want to reconfigure my system for software
RAID and risk screwing something up dreadfully, or reconfiguring things
non-optimally and coming to the wrong conclusion.  Does anyone have a
good idea as to whether my application would likely be significantly
faster using software RAID rather than hardware RAID?

Best wishes to all,

Dale

Sent via Deja.com http://www.deja.com/
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