Re: RAID: identical disks?

On Mon, 28 Nov 2005 11:23:05 -0500, "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.

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.
.

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