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More on RAID .... was .... Re: [K12OSN] Thoughts for my upgrade...



Hi Steve,

> > [...]  I meant RAID5, striped with parity. The capacity then is N-1, 
> > where N is the number of drives in the set. In my case, I have 4 35GB 
> > drives which gives approximately 105GB of storage. 
> 
> 
> I'm no guru, but I understood there had to be an odd-number of drives - 
> generally 3 or 5 ?
> 
> Am I mistaken ?

Yes and no. Those figures would be correct for N= 4 or 6 on RAID5

Basic capacity formulas

RAID 0 = N x Capacity (stripe no parity)
RAID 1 = N x Cap / 2  (mirroring)
RAID 4 = (N-1) x Cap  (Stripe with parity drive)
RAID 5 = (N-1) x Cap  (Stripe with distributed parity)


(As a little aside for those who want to stretch their brains.
 Each RAID type has advantages and disadvantages.
 Each RAID type is best suited to a particular use.
 Which RAID type has the best performance?
 Which RAID type provides the best fault tolerance?
 Which RAID type gives the greatest storage capacity? 
 One of these RAID types is not widely used, why?)

The first non-technical person with the correct answers will get a
chocolate fish (a NZ delicacy, it is not what it seems)! 
Steve you can get your chocolate fish from the Mr. Whippy van! ;-)



Therefore a combination like 

RAID 0+1 of 6 x 1GB drives would give 3 GB total capacity.

Why?

RAID 0
2 stripe sets of 3 x 1GB drives = 2 x 3GB for  or 6GB
then
RAID 1
mirror the two sets = Total capacity of drives / 2 
                    = 6GB                      / 2 
                    = 3GB max capacity

This combination has good performance and reasonable fault tolerance.
BUT
if 1 drive in each stripe set dies simultaneously, YUK!

The preferred option would be RAID 5 + 1. 
Using the drive combination from above, the total capacity would be 2GB.


Different RAID types will have an influence on 3 areas related to your
data.

They are - 
Performance / file access times
Data Integrity
Storage capacity

The trick is to balance these factors to suit your particular
requirements.

But I'd better stop here.

These options are best implemented through hardware anyway.

Here endeth the lesson. ;-)

Cheers,
Bert





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