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42 Using Special Features in Libraries
trade-offs between asynchronous notification and Get Drive Status polling to
detect the listed conditions. In general, we recommend using asynchronous
notification in designs that use a low Get Drive Status polling frequency.
Specifically, we recommend using asynchronous notification for conditions
that the automation controller wishes to detect more quickly than one-half of
the Get Drive Status polling period. See “Recommended Get Drive Status
Polling Frequency” on page 33 for a discussion of the Get Drive Status polling
period.
Configurable Response Period
This configurable mechanism allows the automation controller to limit the time
the tape drive takes to respond to a command.
Typically, a tape drive will exhibit a degree of variability in the time it takes to
respond to any given command. The variation occurs because of processing
dependencies with other events and microprocessor bandwidth limitations in
the tape drive. In extreme cases, the tape drive may require several seconds
to respond to some commands (see “Recommended ACI Time-out Values” on
page 30).
Automation controller designs that use a single microprocessor to
communicate with multiple tape drives via a multiplexed connection may
require tape drive responses within a fixed period. To meet that requirement,
ACI 4.1 includes the configurable response period mechanism.
The response period mechanism allows the automation controller to limit the
response period (that is, the maximum response time) in increments of
100 milliseconds. When a command exceeds the configured response
period, the tape drive will respond to the command before command
processing completes. The status field in the response packet indicates
whether the command has caused a change in the state of the tape drive.
The automation controller may configure the response period using the Set
Drive Configuration command.
Preservation of Drive or Media Error
If the tape drive experiences a hardware fault or a media fault, it will preserve
Error Information that describes the fault condition until the drive process a
Get Error Info command, a cartridge load occurs, or a tape drive reset
occurs. Use the Get Error Info command to access the error information. The
command returns a SCSI Sense Key, Additional Sense Code, Additional
Sense Code Qualifier, and a proprietary error code.