Re: RAID: identical disks?

Curious George <cg@xxxxxxxxx> wrote
> Rod Speed rod_speed@xxxxxxxxx wrote
>> Curious George <cg@xxxxxxxxx> wrote
>>> J. Clarke <jclarke.usenet@xxxxxxxxxxxxxxxx> wrote
>>>> void@xxxxxxxxxxx wrote

>>>>> I've read that it's best to set up RAID with identical disks
>>>>> (same brand and model). I've got a Samsung SP1213N
>>>>> hard drive which I got 21 months ago, and now I want to
>>>>> set up RAID 1 on my machine. So I'll need to get another
>>>>> Samsung SP1213N. But I know that sometimes manufacturers
>>>>> will make slight changes to hardware and still keep the same
>>>>> model number. I don't know if Samsung has done anything
>>>>> to this particular drive in the 21 months since I bought mine, so
>>>>> hopefully buying a recent one will be OK to set up a RAID mirror.

>>>> In practice, as long as the secondary is of the
>>>> same capacity or higher than the primary, and of
>>>> about the same performance, a RAID1 will work fine.

>>> When disks are mismatched the best case scenario is performance,
>>> space, firmware optimizations are limited by the lesser drive. In a
>>> worst case scenario it causes compatibility problems. Fortunately
>>> on modern hardware esp with software or firmware assisted
>>> software raid this worst case scenario is virtually a non-issue.

>>>> In fact there is a school of thought that the two disks should
>>>> be different brands and/or models on the theory that if they
>>>> are they same they might be so closely matched that they
>>>> both might fail at the same time or close enough to it that
>>>> you don't have time to rebuild the mirror.

>>>> Having seen four Japanese-made light bulbs installed at the
>>>> same time fail within four hours of each other a year and a
>>>> half later, I am not going to denigrate this notion, although
>>>> I don't consider such failure to be exceedingly likely.

>>> There is most definitely a U-shaped or bathtub curve to hardware
>>> failure over time. However both drives in a 2 drive array dying
>>> natural deaths within hours of another is quite unlikely.

>> Not if the system is out of spec, too hot or a bad power supply.

> that wouldn't be a "natural death" now would it

The original wasnt about "natural death", it was clearly
about what gives the best result, regardless of what
happens. It aint just about handling hard drive failure
gracefully, its also about handling other failures
gracefully too if that can be done essentially for free
by using two different drives instead of two identicals.

>>> Rather than mixing models some ppl buy parts from
>>> different suppliers to hedge their bets or simply count
>>> on premature failure to introduce media of different
>>> ages or simply proactively decommission arrays at
>>> the end of expected service life rather than wait for
>>> the catastrophic event.

>> Or just use two quite different models that are compatible.

> offering an alternative is not saying "you can't."

Never said you did, I just pointed out the advantage of that approach.


.

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