Fibre Channel Directors: Myths, Realities, and Evaluations Page 4
Is a Director-class Switch Needed?
Most sites using director-class switches are doing so because they have both very high port count and very high reliability requirements. The high port count for director switches allows for simplified management, and the reliability compared with non-director-class switches speaks for itself. Of course, the cost is greater per port with director-class switches, but the costs associated with downtime and the time needed for personnel to manage many smaller switches are expensive as well.
The other major reason director-class switches are used is in large configurations of servers, HBAs, and RAID devices that require high performance. Given that enterprise RAIDs from EMC, HDS, and IBM support as many as 64 connections, it's not necessary to use director-class switches unless the configurations are large or for mid-range RAIDs that have only a few number of ports and failover requirements. Consider the following example:
This is a common HA (High Availability) configuration used with mid-range RAID controllers that support active/passive failover. Of course, this is a very small configuration and could be dramatically expanded. Keep in mind that an HBA's reliability based on BELLCORE numbers (a standard reliability calculation) is usually between 250K and 400K hours. So with 32 HBAs and using the 250K hour term, you can expect 1 failure per year. This means that unless you have hotswap PCI/PCI-X slots, you will need to take the server down.
Taking all of this into account, I believe it is a good idea to consider a director-class switch if:
- You need very high reliability
- Your downtime requirements for upgrades are very narrow and the director-class switches allow hot upgrades
- You have a large port count requirement
- You have limited personnel that can manage and upgrade many small switches
If you need very high availability access to the storage, the above configuration using two director-class switches connected to the storage system (whether it is mid-range RAID or enterprise RAID) should exceed the 5 9s of uptime and data availability.