ABCs of z/OS System Programming Volume 3 - IBM Redbooks

8.11 SSA loops
SSA operation
4 links per loop
2 read and 2 write
simultaneously in each direction
40 MB/sec on each link
Loop availability
Loop reconfigures itself
dynamically
Spatial reuse
Up to 8 simultaneous
operations to local group of
disks (domains) per loop
Figure 8-11 SSA loops
SSA operation
SSA is a high performance, serial connection technology for disk drives. SSA is a full-duplex
loop-based architecture, with two physical read paths and two physical write paths to every
disk attached to the loop. Data is sent from the adapter card to the first disk on the loop and
then passed around the loop by the disks until it arrives at the target disk. Unlike bus-based
designs, which reserve the whole bus for data transfer, SSA only uses the part of the loop
between adjacent disks for data transfer. This means that many simultaneous data transfers
can take place on an SSA loop, and it is one of the main reasons that SSA performs so much
better than SCSI. This simultaneous transfer capability is known as “spatial release.”
Each read or write path on the loop operates at 40 MBps, providing a total loop bandwidth of
160 MBps.
Loop availability
The loop is a self-configuring, self-repairing design that allows genuine hot-plugging. If the
loop breaks for any reason, then the adapter card will automatically reconfigure the loop into
two single loops. In the ESS, the most likely scenario for a broken loop is if the actual disk
drive interface electronics fails. If this happens, the adapter card dynamically reconfigures the
loop into two single loops, effectively isolating the failed disk. If the disk is part of a RAID
array, the adapter card will automatically regenerate the missing disk using the remaining
data and parity disks to the spare disk. After the failed disk is replaced, the loop is
automatically reconfigured into full duplex operation and the replaced disk becomes a new
spare.
464 ABCs of z/OS System Programming Volume 3
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