RAID-1 reliability

I'm going to set up a small departmental server for a customer of us.
I consider setting up a RAID-1 system.
This isn't my first RAID, but I don't have a wide experience on these
solutions, and I would like to hear the opinion of others on a couple of
points.

I am rather suspicious about RAID-1 real reliability.
It's often said that it's best if the disks are the same model & same size.
However, using two drives with the same manufacturing standards, the same
tolerances, the same MTBF, etc. etc. increases the chance that both fail
simultaneously.

Did someone of you experience such failures?
To which extent may I deviate from the above rule (same model-same size
disks)?

_I apologize for my English_
Thanks
Marco.

marcodeo wrote in
message ...

I'm going to set up a small departmental server for a
customer of us. I consider setting up a RAID-1 system.

This isn't my first RAID, but I don't have a wide
experience on these solutions, and I would like to
hear the opinion of others on a couple of points.

RAID-5 is there for a reason.

I am rather suspicious about RAID-1 real reliability.

It's often said that it's best if the disks are the same model & same size.

Not for that reason. Thats for performance reasons.

However, using two drives with the same manufacturing
standards, the same tolerances, the same MTBF, etc.
etc. increases the chance that both fail simultaneously.

Nope, it does not.

Did someone of you experience such failures?

To which extent may I deviate from the
above rule (same model-same size disks)?

You may not get as good performance.

Previously marcodeo wrote:
I'm going to set up a small departmental server for a customer of us.
I consider setting up a RAID-1 system.
This isn't my first RAID, but I don't have a wide experience on these
solutions, and I would like to hear the opinion of others on a couple of
points.

I am rather suspicious about RAID-1 real reliability.
It's often said that it's best if the disks are the same model & same size.
However, using two drives with the same manufacturing standards, the same
tolerances, the same MTBF, etc. etc. increases the chance that both fail
simultaneously.

Not really. What happens is that nonrandom failures can happen
in the same way on both disks, if there are design problems with
the disks. For random failures RAID-1 is just as reliable with
similar disks as with different ones. But even design errors
will in many cases not lead to simultaneous failures. Personally
I think it is important to be able to replace a failed disk fast.

The only real risk IMO is something that really leads to simultaneous
failure, such as high sensibility to overtemperature, overvoltage or
physical shock.


Did someone of you experience such failures?
To which extent may I deviate from the above rule (same model-same size
disks)?

Usually you can combine arbitrary disks. You will only get
the capacity of the smaller one though.

Arno

_I apologize for my English_

Why? Seems fine to me.

Thanks
Marco.

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On Wed, 27 Aug 2003 06:07:39 +1000, "Rod Speed"
wrote:

However, using two drives with the same manufacturing
standards, the same tolerances, the same MTBF, etc.
etc. increases the chance that both fail simultaneously.

Nope, it does not.

I think it does.

You're wrong.

Or it's just by luck that we get several disks of the
same models failing the same day arriving at our lab.

You'd get the same result if you received
shipments of mixed drives instead.



What I mean is that we usually receive hard drives from the same
manufacturer, same model from customers all over Europe to be repaired
the same day. When it's once you can call it coincidence, when it's
once a week it's a bit too often, especially when it's exactly the
same failure for the drives.

Nick

On Tue, 26 Aug 2003 22:40:50 +0200
Nick dot fr wrote:

On Wed, 27 Aug 2003 06:07:39 +1000, "Rod Speed"
wrote:

However, using two drives with the same manufacturing
standards, the same tolerances, the same MTBF, etc.
etc. increases the chance that both fail simultaneously.

Nope, it does not.

I think it does.

You're wrong.

Or it's just by luck that we get several disks of the
same models failing the same day arriving at our lab.

You'd get the same result if you received
shipments of mixed drives instead.



What I mean is that we usually receive hard drives from the same
manufacturer, same model from customers all over Europe to be repaired
the same day. When it's once you can call it coincidence, when it's
once a week it's a bit too often, especially when it's exactly the
same failure for the drives.

Were the manufacturing dates on the drives the same?

Nick


--
--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)

Nick dot fr wrote in message
...
Rod Speed wrote

However, using two drives with the same manufacturing
standards, the same tolerances, the same MTBF, etc.
etc. increases the chance that both fail simultaneously.

Nope, it does not.

I think it does.

You're wrong.

Or it's just by luck that we get several disks of the
same models failing the same day arriving at our lab.

You'd get the same result if you received
shipments of mixed drives instead.

What I mean is that we usually receive hard
drives from the same manufacturer, same model from
customers all over Europe to be repaired the same day.

Dont believe it.

When it's once you can call it coincidence, when
it's once a week it's a bit too often, especially
when it's exactly the same failure for the drives.

Dont believe you get that effect with every
drive model from every hard drive manufacturer.

You may get that result with just one drive model
like say the Fujitsu MPGs, but thats just a design
problem and even you must have noticed that all
MPG drives in europe dont all die on a single day.

"Rod Speed" baited:

Dont believe it.

Dont believe you get that effect with every
drive model from every hard drive manufacturer.

You may get that result with just one drive model
like say the Fujitsu MPGs, but thats just a design
problem and even you must have noticed that all
MPG drives in europe dont all die on a single day.


Don't even bother to answer that bait, Nick.
Let one of his fake "minions" answer it.


*TimDaniels*

Some pathetic little ****wit claiming to be
Timothy Daniels wrote in
message ...
just the puerile silly **** thats all it can ever manage.

On Wed, 27 Aug 2003 22:45:04 +0200
Nick dot fr wrote:

On Thu, 28 Aug 2003 06:07:37 +1000, "Rod Speed"
wrote:

You clearly havent manage to grasp
even the most basic statistical concepts.

Sorry, I only have an MSc in Pattern Analysis, and I have a very
strong Bayesian background.

You should have a look at Renewal Theory, 1962 from Cox, you would
learn one or two thing about time of failure and so on.
If a model as a flaw in its conception, it's very very likely that
most of the drives will fails in a very short period. The distribution
of time of failure is an Erlangian distribution, with a peak of
failure where most of the drives fail. The pdf of the distribution is
(rho^alpha * t^alpha * exp (-rho * t) )/ gamma(rho)

where alpha and rho are parameters, and t the time. One can see that
there is a much more likely time t0 of failure than any others


Add this with Murphy's law, and you'll be likely to have the first
disk crashing when nobodys here, and the second one when the guy in
charge arrives to change the first one.

What you're describing sounds like infant mortality, not the failures
that occur in normal long-term operation.



Nick


--
--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)

On Wed, 27 Aug 2003 07:41:16 +1000, "Rod Speed"
wrote:


Dont believe you get that effect with every
drive model from every hard drive manufacturer.

You may get that result with just one drive model
like say the Fujitsu MPGs, but thats just a design
problem and even you must have noticed that all
MPG drives in europe dont all die on a single day.


Obviously, but when you discover too late that there's been a flaw in
the conception of the drive, you're in trouble.
The best exemple I have is with Quantum Fireball AS Plus. We never
receive only one of them, but rather pairs or triple, same model, same
date, and same failure (chip burned, or one platter not recognized any
more)

Nick