Big Iron Secrets

The consumer-grade market has edged closer to SAS for a little while now but still lacks Fibre Channel Arbitrated Loop (FC-AL), which is commong in the enterprise chassis sector. Most high-end SAN environments use FC-AL as the primary means of Tier1 storage, migrating down to SAS or SATA, or Tape. FC-AL offers redundancy in spades. With the advent of 6Gbit/sec SAS, FC-AL may have a Tier1 competitor however.

Finistar shortwave 4Gbit/sec SFP, turning light into 0s and 1s

A quick example of enterprise disk redundancy… You have a 15k RPM FC-AL disk array and a SATA array. A fibre path goes dark (a loss of connectivity on the fibre link). You have lost communications to your SATA disk pack. The same thing happens to the FC-AL tray. It doesn’t matter however, as the FC-AL interface is multi-pathed (sometimes referred to as MPxIO). You can destroy:

A fibre communication path to your disk.
A chassis controller.
A physical disk module onboard controller (yes, the green PCB connections on the disk!).

...and you still maintain availability.

With such complicated disk controller technology, very comprehensive configuration mechanisms are required to appropriately administer such environments. Array management software, such as Dell’s PowerManage or Sun’s Common Array Manager are used to configure Logical Unit Numbers (LUNs), World Wide Names (WWNs) and domain partitioning of virtual disks.

Common Array Manager virtual disk configuration

Disk allocation/volume configuration tools

Virtual disk volumes from enterprise disk controllers are presented to HBAs via WWNs associated with the controller part number and disk type.