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

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DRAFT -- Revision 2.0
August 22, 1996Page 21
failure of the primary path (host adapter, cable or storage processor), LVM will
automatically switch to the redundant path.
The
PV Links
feature is necessary to support the redundant storage processor
available on both the Model 10 and Model 20. This feature causes LVM to switch to the
hardware path of the redundant storage processor when the corresponding primary
storage processor fails. Note that the disk arrays
must
be configured with LVM to make
use of this automatic switching feature. Figure 5 shows how the
PV Links
feature is
used with dual storage processors. Use of the redundant device file name without LVM
can result in severe performance degradation due to the several second delay
associated with storage processor switching in the HADAs.
The recommended number of HADAs per F/W SCSI interface changes depending
upon the I/O load and upon the number of
active
storage processors. For example,
with two active storage processors in the Model 20 and a heavy I/O load, only four
Model 20 HADAs are recommended on a given F/W SCSI interface, although seven
would be theoretically supported. This configuration also assumes that each of the two
storage processors is connected to a different F/W SCSI host adapter. The
consequence of exceeding the recommended number of HADAs per F/W SCSI link is
reduced overall performance of the application. Performance and capacity are
discussed in greater detail later.
Figure 5 shows the only supported configuration for HADAs with two Storage
Processors when configured in an HA cluster. This configuration was established for
both performance and availability reasons. There are no SPOFs in this configuration
since each storage processor is attached to a different SCSI bus and host adapter.
The combination of
PV Links
and dual storage processors with HADAs provides very
similar availability as JBODs in association with the HA software MC/ServiceGuard and
MC/LockManager.