[KLUG Hardware] Re: 3rd party reviews / RAID

[Bryan J. Smith]

On Wed, 2002-05-22 at 09:06, Mike Williams wrote:
> Most of the stuff I've read recently, from MaximumPC and others, suggests
> that IDE RAID 0 doesn't gain you much, at least with the Promise and such
> controllers.

It doesn't give you crap over the OS' software RAID-0.  In fact, if you
use slaves with masters on the same channel, it can be worse.

> I seem to remember somebody (Adam?) ranting against them as
> not even being true RAID cards but regular IDE controllers being sneaky.

Me.  Of course, there _are_ "real" ATA RAID controllers, but only 3
products.  The Adaptec 2400A, Promise SuperTrak and the 3Ware Escalade.

I'm writing an article for a major magazine right now and need
reviewers.  Let me know if you are intersted.

> I built my current system with a Promise RAID 0 array, but I'm kinda
> regretting it.  Performance increase is minimal, and I know I've opened
> myself up to roughly double the possibility of catastrophic hard drive
> failure.

I have a 3Ware Escalade 6410 in my current, dual-Athlon workstation. 
I've only got two 80GB disks in RAID-0 right now (160GB usable), but I'm
going to go to four 80GB disks in RAID-10 (still 160GB usable) which
will give me even greater read performance.

In my server, a dual-Celeron, I have a 3Ware Escalade 7800.  It has two
80GB disks in RAID-1 (80GB usable) and six (yes 6) 40GB/7200rpm disks in
RAID-5 (200GB usable).  It kicks serious butt.

> Somebody correct me if I'm wrong, but in general, RAID 0 can actually
> slow you down.  In reading, which is the majority of hard drive
> activity unless you're editing video or something, rotational latency
> is the biggest speed factor.
> Rotational latency being the time you have to wait for the piece
> of data you're interested in coming around to the read head.  In a stripe
> set, you have to wait for the data to come around on both drives, which
> increases the odds that you'll be waiting closer to the maximum time.

If you are using the _exact_same_drives_, the latency is NOT increased.

In fact, several SCSI drive models have a line for maintaining the same
rotational position or "spindle synchronization" to combat this.  But
most have found it makes little difference in modern drives.

The additional reality is that _real_ (i.e. non-Win/DOS) OSes queue
writes and order them as sequentially as they can.

> SCSI RAID arrays have always been able to synchronize the drives

Er, AFAIK, *NO* SCSI protocol spec does this!  The few drives that do
are vendor-specific, model-specific.

> so they rotate together, but I've never heard of such a thing in IDE.

Again, if you are using the same models, spindle synchronization is not
that big of an issue in modern drives.  They usually seek near
synchronous without any additional logic/cabling.

-- Bryan

-- 
The US government could be 100x more effective, and 1/100th the
Constitutional worry, if it dictated its policy to Microsoft as
THE MAJOR CUSTOMER it is, and not THE REGULATOR it fails to be.
---------------------------------------------------------------
Bryan J. Smith, SmithConcepts, Inc.   mailto:b.j.smith@ieee.org
Engineers and IT Professionals     http://www.SmithConcepts.com