The maximum allowable single fabric is 239 switches per fabric. Most fabrics are created by using 16 or fewer switches in order to keep fabric re-config times down to a minimum. Although tests have been done with over 44 switches (domains) in a single fabric, it's usually best to use more smaller fabrics. When the SAN starts growing and more switches are needed, it's usually time to start thinking about director class switches.
There are directors available from 64 ports all the way up to 256 ports using 16 256-port directors in a single fabric, bringing your port count up to 4,096 ports!
You do not have to connect all the switches into a large fabric to manage multiple SAN islands. As long as you have an IP connection between the devices in the SAN islands all switches and storage can be managed from a single console. This brings up how many "network of SANs there are in the US and the trend.
As I mentioned before, best practice means using many smaller fabrics managed together as a single entity through a SAM (Storage Area Management) software solution. MOST of the large corporations I have been to use this approach. So the trend I see is to create SWANS (Storage Wide Area Networks).
The interconnect between islands needs to be considered when connecting multiple SAN islands together. There will be an IP connection used for access by the SAM software for storage management and there will be a Fibre Channel connection to tie the islands together into larger SANs. The Fibre Channel connection can be, depending on distance:
* A switch-to-switch ISL link via multi-mode or single-mode optical fiber
* A leased dark fibre connection through a Telco
* A Dense Wave Division Multiplexed link (DWDM)
* An iSCSI link through an iSCSI gateway over IP
* An IP link using either the FCIP or iFCP protocols
The link you choose will be based on your budget, the distance between sites, the bandwidth required and the switch count within each fabric at each SAN island. If you are trying to tie together two fabrics into one bigger one, you need to also be concerned with naming conventions. If you have the same zone name or alias name being used in both fabrics, the fabrics will segregate and you will not be able to merge the fabrics. If you have many switches at each site and use a SAN extender that uses a tunneling protocol like FCIP, you will be creating one large fabric and all error traffic and re-configuration will be propagated across the link. iFCP will connect two SAN islands, while keeping all error traffic isolated within each SAN island. iFCP will allow you to connect up many SAN islands without worrying about errors or re-configuration issues.
I will be doing a webcast on the SearchStorage.com site in the very near future that discusses all these issues in much greater detail. You can also get more information from chapters 6,7 and 8 of my book "Storage Area Networks for Dummies." Chapter 8 discusses "networking SANs" in detail.
Chris
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