What is Rule of 17? 1

[samtaf]

My question is what exactly rule of 17 is. In most forums, this question is answered vaguely just by saying, its EMC's mathod of redundancy.
as we know, All SAN environment are built in rudundant fabric,and storage evices are also attached to the SAN likewise. What is the role of Rule of 17 here?
Why is it only EMC uses it, not IBM, not HP?
Please explain by defining the term also.

Thanks

 

[denisfr]

Hello,

In a SAN product (HBA, switch, director...), data is processed by a specific chip (called ASIC) throught a specific connection chain (including power).
But for cost reasons (and also processing latency), there is not one chip dedicated to a single role.
So, in a EMC product like a disk array, you can find, for example, 8 chips for 16 FC cards.
If you want to plug 2 fibers, you must be sure that your data will not be processed by the same ASIC, in case of lost of this ASIC.
So, if you use the 'rule of 17', by plugging a fiber on the 3rd and on the 14th FC card you are sure that your 2 fibers are connected to cards which are processed by 2 ASICs.
I took the ASIC example, but in real, it is the entire connection chain which is secured by the rule of 17...

This redundancy method is the one used by EMC.

IBM has other things to result in the same security.

More clear, now ?

Denis