[SunRescue] Excuse my gross oversimplification of RAID

[Paul Theodoropoulos]

At 02:28 PM 12/2/00, you wrote:
>I do take issue with the exception Paul Theodoros made for my Raid 
>5
>explanation in calling it striping with rotated parity. While this 
>is true,
>it is not generally referred to explicitly as having rotated 
>parity. RAID 5
>could also, and possibly more accurately referred to as having 
>distributed
>parity, because the decision of where to put the parity chunk is 
>up to the
>vendors implementation, and it is not necessarily always rotated, 
>it may be
>distributed across an array to optimize access patterns if an 
>intelligent
>RAID controller is being used.

good points. my choice of 'rotated' was poor - 'distributed' is 
clearly the more descriptive term, and is the specific quality that 
differentiates RAID 5 from the other parity-based redundancy 
schemes, which rely upon dedicated ('non-distributed') parity 
drives.

An additional observation, the issue of 'hot spares' is not even 
touched upon in the RAID specification, however it's use is 
probably one of the most significant steps one can take in assuring 
the safety of one's data in a RAID scheme. In any redundancy 
scheme, one or more hot-spares can increase the reliability and 
decrease the MTTF by an order of magnitude. Most 
reasonably-intelligent controllers can rebuild data from a failed 
disk onto a hot-spare in a matter of a few hours (highly dependent 
upon current load on other disks of course). if a disk fails in the 
middle of the night when nobody is around, it's fabulous to be able 
to turn off the beeping pager, sound in the knowledge that you can 
swap out the bad disk in the morning - at your leisure, rather than 
rushing in a panic to the data center to swap it out ASAP, for fear 
that another disk may die in that window.

hmm, does it sound like i'm speaking from experience? ;^)


-----------------------------------
Paul Theodoropoulos   paul at atgi.net
Senior Unix Systems Administrator
Advanced Telcom Group, Inc.
Santa Rosa, California
Work: http://www.atgi.net
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