Chapter 4 Mass Storage Subsystem Configuration 83
Dual-Loop Configurations
The FC-AL disk backplane provides dual-loop access to all internal disk drives, in
both full and split backplane configurations (see “Full vs. Split Backplane
Configurations” on page 83). Dual-loop configurations enable each disk drive to be
accessed through two separate and distinct data paths. This capability provides:
■ Increased bandwidth – Allowing data transfer rates up to 200 Mbytes per second
versus 100 Mbytes per second for single-loop configurations
■ Hardware redundancy – Providing the ability to sustain component failures in one
path by switching all data transfers to an alternate path
For more information on the RAS and performance capabilities of the mass storage
subsystem, see “About Sun Fire V890 Mass Storage Features” on page 79.
Note – To take advantage of the dual-loop capability of the FC-AL backplanes, an
optional PCI FC-AL host adapter card must be installed to control the second loop
(Loop B). For more information, see “About FC-AL Host Adapters” on page 86.
Port bypass controllers (PBCs) on the disk backplane ensure loop integrity. When a
disk or external device is unplugged or fails, the PBCs automatically bypass the
device, closing the loop to maintain data availability.
Each backplane also includes two integrated SSC100 SCSI Enclosure Services (SES)
controllers, one for each loop. The SES controllers:
■ Interpret enclosure service commands from the host software
■ Manage the backplane’s FC-AL loop configuration
■ Monitor status signals from disks and disk backplanes
■ Control disk status LEDs
When a second backplane is installed to form an expanded 12-disk array, only the
SES controllers on the base backplane are active; the SES controllers on the upper
backplane remain inactive.
Full vs. Split Backplane Configurations
In the expanded 12-disk array, the two FC-AL disk backplanes are connected by
internal cables. These cables extend Loop A (and optionally, Loop B) between the
two backplanes. This is sometimes called a full backplane configuration. The dual-loop,
full backplane configuration provides redundancy of disks, data paths, and host
adapters.
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