StorageTek Host Software Component 6.0, Руководство системного программиста

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30 VM/HSC 6.0 System Programmer’s Guide
1st ed., 6/30/04 - 312579601
Precedence of VOLATTR and TAPEREQ Statements
The precedence of VOLATTR and TAPEREQ statements depends on whether the request
is for a specific or nonspecific volume.
Specific Volume Requests
For a specific volume request, VOLATTR information overrides TAPEREQ information
provided that the VOLATTR statements supply both media type and recording technique.
For example, if the TAPEREQ statement specifies MEDia(ECART) RECtech(36track)
and the VOLATTR statement specifies MEDia(Standard) but does not specify RECtech,
the result is MEDia(Standard) from the VOLATTR statement and RECtech(36track) from
the TAPEREQ statement.
If the VOLATTR statement provides a value for RECtech, then that recording technique is
used, but the TAPEREQ statement can ‘‘refine’’ the RECtech value. For example, the
VOLATTR statement specifies a recording technique of 36track, and the TAPEREQ
statement specifies 36Btrack, then 36Btrack is used.
Nonspecific Volume Requests
Note: When mixing media types, the user must define VOLATTR statements because
scratch counts are determined entirely from VOLATTR information.Accurately defined
VOLATTR statements are critical for correct processing of nonspecific volume requests.
The HSC analyzes the TAPEREQ information to determine the required media type and
recording technique, and then tries to locate available scratch volumes in the library that
match these values. If no matching scratch volumes are available, HSC rejects the request.
Scratch Selection
Scratch volumes are segregated by means of scratch subpooling with VOLATTR
statements. Assuming that requested subpool definitions do not conflict with the
applicable VOLATTR statements, the HSC provides the following services to satisfy a
request:
• If scratch volumes are segregated by defining subpools and specifying VOLATTR
statements, the HSC ensures that a volume with the appropriate media type is
selected from the correct subpool.
• If scratch volumes are segregated only by defining subpools, the HSC selects a
standard cartridge from the correct subpool. A standard cartridge is selected because
this is the default when VOLATTR statements are not defined.
• If volumes are segregated only by specifying VOLATTR statements, the HSC
ensures that a volume with the appropriate media type is selected.