[Gllug] re: software vs hardware RAID

[Rich Walker]

Christopher Hunter <chrisehunter at blueyonder.co.uk> writes:

> On Thursday 04 Nov 2004 15:04, Martin A. Brooks wrote:
>
>> Then you miss the point of RAID entirely.  RAID has _never_ been about
>> stopping you losing data.  It's about speed and redundancy.
>
> Umm...  no.  The primary objective of RAID is to make disk failures less 
> catastrophic.  That sounds like prevention of loss of data to me!  Any speed 
> benefits are just a happy side-effect.

Given that RAID-0 is a valid raid array, that can't be the case :->

The original objectives for RAID were to increase speed of the system
without increasing MTBF. N drives with striping clearly increases speed
N-fold, but does terrible things to MTBF.

Besides, disk failures come in three major classes:

1. It Just Broke.

2. They All Just Broke.

3. Whoops, Someone Nuked the Datacenter.

Usually, the first two feature someone holding a piece of the computer
with a silly grin on their face. The third will have a Texan with a
silly grin involved.

RAID>0 protects against the first class.

Backups protect against the second class.

Geographical Survivability (otherwise known as "put the other half of
the cluster 500 miles away[1], in a different legal jurisdiction") protects
against the third class. (And if it doesn't - you probably don't have
staff left to worry about the backups :->)

cheers, Rich.




>
> Chris 
> -- 
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Footnotes: 
[1]  Apparently, the limits on the VAX cluster interconnect technology
     were set at one point by the need of banks in New York to have the
     "other half" of the cluster outside US jurisdiction...

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