Choosing the Right Disk Technology in a High Availability Environment DRAFT Version 2.0, August 1996

DRAFT -- Revision 2.0
August 22, 1996Page 38
missing data that resides on the failed mechanism. This situation will severely degrade
the overall performance of the disk array.
Disk Link Technology
The various disk link technologies have different peak and sustained bandwidths.
Obviously, a faster bandwidth link will provide better performance. Currently, F/W SCSI
offers the highest bandwidth and therefore the best performance.
Number of Targets on a Disk Link
The bandwidth and target limitations of the various disk link technologies was discussed
in an earlier section. Each disk link technology has a physical limit on the number of
targets allowed. This limit is based on electrical and protocol requirements and varies
from 8 to 16. However, in a real environment, performance requirements will often
dictate that fewer targets be configured on a particular disk link, thus requiring that
additional disk links be configured in order to meet the total storage requirements. The
number of disk links is constrained by the number of I/O slots in a given SPU and upon
testing and other support limitations.
Furthermore, in multi-initiator bus environments, fewer targets may be configured on a
disk link when each of the initiators is active on the bus. It is attractive for cost reasons
to configure HA clusters such that disks belonging to different applications are
configured on the same bus. These applications could be running on different SPUs
and it is this case that degrades performance the most. Therefore, it is highly
recommended that disks on a particular bus belong to only one application.
Following are guidelines on how to configure a disk link for maximum throughput. One
must always consider, though, that one key issue is the number of active disks on a
link, not just the number of disks on a link. The consequence of exceeding these
guidelines is reduced overall application performance due to disk link (bus) saturation;
i.e., some I/Os will be queued up since the bus is busy.
For performance reasons, the maximum number of Performance Loads allowed on
a F/W SCSI bus is 11-1/2. Performance Loads are established for the host adapter as
well as disks, and in a multi-initiator HA environment, each host adaptor must be
counted if it is active on the same bus.
The assigned Performance Load Factor for a particular host adapter, disk, or array is
based on the sustained transfer rate. This definition includes the possibility that
multiple host adapters on different nodes might be accessing the same F/W SCSI bus
concurrently, although this is not recommended. Host adapters should be assigned the
highest priority SCSI addresses in order to minimize starvation.