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EMC: SAN switch fabric topology math
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: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