Re: H/W RAID 5 on slower disks versus no raid on faster HDDs

On Mon, 2002-11-25 at 23:03, David Gilbert wrote:
>
> I'm on a bit of a mission to stamp out this misconception. In my
> testing, all but the most expensive hardware raid controllers are
> actually slower than FreeBSD's software RAID. I've done my tests with
> a variety of controllers with the same data load and the same disks.
>

I agree 100%: hardware raid sucks.
We had a raid 5 Postgres server on midgrade hardware with 5 60gig 7200rpm
IDE disks (240 gig total) and the thouroughput was just as high (maybe 10%
less) than a single disk. Same for the seek times. CPU around 1Ghz never
hit more than 10% for the raid service. Since very few databases are CPU
limited, this is a small price to pay.

We confirmed the performance results with heavy testing. There is virtually
no disadvatage to software raid, just spend 10$ and get a 10% faster cpu.

The linux software raid drivers (and others I assume) are very optimized.
Not to sure about m$ but win2k comes with raid services, its pretty
reasonalbe to believe they work ok.

You can double the performance of a raid system by going 0+x or x+0 (eg 05
or 50). just by adding drives. This really doubles it, and an optmized
software driver improves the seek times too by placing the idle heads it
different places on the disks.

p.s. scsi is a huge waste of money and is no faster than IDE. IMHO, scsi's
only advantage is longer cables. Both interfaces will soon be obsolete with
the coming serial ATA. High rpm disks are very expensive and add latent
heat to your system. Western digitial's IDE disks outperform even top end
scsi disks at a fraction of a cost. You can install a 4 drive 10 raid setup
for the cost of a single 15k rpm scsi disk that will absolutely blow it away
in terms of performance (reliability too).

Just remember, don't add more than one IDE disk on a raid system to a single
IDE channel! Also, do not attempt to buy IDE cables longer than 18"..they
will not be reliable.

Merlin