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Multiple Adapters
Some SSA subsystems (see "Rules for SSA Loops" on page 22) allow a disk drive to be
controlled by up to two adapters in a particular using system. The disk drive has,
therefore, two paths to each using system, and the SSA subsystem can continue to
function if an adapter fails. If an adapter fails or the disk drive cannot be accessed from
the original adapter, the SSA disk device driver switches to the alternative adapter
without returning an error to any working application.
When a disk drive has been successfully opened, takeover by the alternative adapter
does not occur simply because a drive becomes reserved or fenced out. However,
during an open of an SSA logical disk, the device driver does attempt to access the
disk drive through the alternative adapter if the path through the original adapter
experiences reservation conflict or fenced-out status.
A medium error on the disk drive does not cause takeover to occur. Takeover occurs
only after extensive error-recovery activity within the adapter and several retries by the
device driver. Intermittent errors that last for only approximately one second usually do
not cause adapter takeover.
When takeover has successfully occurred and the device driver has accessed the disk
drive through the alternative adapter, the original adapter becomes the standby adapter.
Takeover can, therefore, occur repeatedly from one adapter to another so long as one
takeover event is completed before the next one starts. Completion of a takeover event
is considered to have occurred when the device driver successfully accesses the disk
drive through the alternative adapter.
When takeover has occurred, the device driver continues to use the alternative adapter
to access the disk drive until either the system is rebooted, or takeover occurs back to
the original adapter.
Each time the SSA disks are configured, the SSA disk device driver is informed which
path or paths are available to each disk drive, and which adapter is to be used as the
primary path. By default, primary paths to disk drives are shared equally among the
adapters to balance the load. This static load balancing is performed once, when the
devices are configured for the first time. You can use the chdev command to modify the
primary path.
Because of the dynamic nature of the relationship between SSA adapters and disk
drives, SSA pdisks and hdisks are not children of an adapter but of an SSA router. This
router is called ssar. It does not represent any actual hardware, but exists only to be
the parent device for the SSA logical disks and SSA physical disks.
Note: When the SSA disk device driver switches from using one adapter to using the
other adapter to communicate with a disk, it issues a command that breaks any
SSA-SCSI reserve condition that might exist on that disk. The reservation break
is performed only if this using system has successfully reserved the disk drive
through the original adapter. This check is to prevent adapter takeover from
breaking reservations that are held by other using systems. If multiple using
systems are connected to the SSA disks, SSA-SCSI reserve should not,
therefore, be used as the only method for controlling access to the SSA disks.
Chapter 13. Using the Programming Interface
267
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