EMC: SAN switch fabric topology math

Title:

EMC: SAN switch fabric topology math

Author:

Douglas O’Leary <dkoleary@olearycomputers.com>

Description:

Math demonstrating port gains by adding switches

Date created:

Date updated:

Disclaimer:

Standard: Use the information that follows at your own risk. If you screw up a system, don’t blame it on me…

EMC recommends no more than four connectrix switches per fabric based on the following formulae:

	One Switch		Two Switch
32	Total ports	64	Total ports
4	ports reserved for card failure	4	ports reserved for card failure
28	ports remaining.	4	ports reserved for E_ports
(int(28/5))	No more than 4:1 ratio, hosts : fa	56	ports remaining.
23	Possible host connections	(int(56/5))	No more than 4:1 ratio, hosts : fa
/ 2	to support multi-pathing	45	Possible host connections
11	total host connections	/ 2	to support multi-pathing
		22	host connections (gain of 11)
	Three switches		Four switches
96	total ports	128	total ports
4	ports reserved for card failure	4	ports reserved for card failure
12	ports reserved for E_ports	24	ports reserved for E_ports
80	ports remaining	100	ports remaining
(int(80/5))	No more than 4:1 ratio, hosts : fa	(int(100/5))	No more than 4:1 ratio, hosts : fa
64	Possible host connections	80	Possible host connections
/ 2	to support multi-pathing	/ 2	to support multi-pathing
32	host connections (gain of 10)	40	host connections (gain of 8)

Putting in that fourth connectrix means that you gain only 8 host connections from a 32 port connectrix switch. At EMC prices, that’s going to be a hard sell to the resident bean counters