Guardian Procedure Calls Reference Manual (G06.25+)

Guardian Procedure Calls (R)
Guardian Procedure Calls Reference Manual—522629-013
13-8
READ[X] Procedures
Disk File Considerations
•
Large data transfers for unstructured files using default mode
For the read procedures (READ[UPDATE] [LOCK] [X]), using default mode allows
I/O sizes for unstructured files to be as large as 56 kilobytes (57,344), if the
unstructured buffer size is 4 KB (4096). Default mode here refers to the mode of
the file if SETMODE function 141 is not invoked.
For an unstructured file with an unstructured buffer size other than 4 KB, DP2
automatically adjusts the unstructured buffer size to 4 KB, if possible, when an I/O
larger than 4KB is attempted. However, this adjustment is not possible for files that
have extents with an odd number of pages; in such cases an I/O over 4 KB is not
possible. Note that the switch to a different unstructured buffer size will have a
transient performance impact, so it is recommended that the size be initially set to
4 KB, which is the default. Transfer sizes over 4 KB are not supported in default
mode for unstructured access to structured files.
•
Large data transfers using SETMODE 141
For READX only, large data transfers (more than 4096 bytes) can be done for
unstructured access to structured or unstructured files, regardless of unstructured
buffer size, by using SETMODE function 141. When SETMODE 141 is used to
enable large data transfers, it is permitted to specify up to 56K (57344) bytes for
the
read-count parameter. Refer to Table 14-4 for use of SETMODE function
141.
•
Structured files
•
a subset of records for sequential READ[X]s
The subset of records read by a series of calls to READ[X] is specified through
the POSITION or KEYPOSITION procedures.
•
reading of an approximate subset of records
If an approximate subset is being read, the first record returned is the one
whose key field, as indicated by the current key specifier, contains a value
equal to or greater than the current key. Subsequent reading of the subset
returns successive records until the last record in the file is read (an EOF
indication is then returned).
•
reading of a generic subset of records
If a generic subset is being read, the first record returned is the one whose key
field, as designated by the current-key specifier, contains a value equal to the
current key for
comparison-length bytes. Subsequent reading of the file
returns successive records whose key matches the current key (for
comparison-length bytes). When the current key no longer matches, an
EOF indication returns.
For relative and entry-sequenced files, a generic subset of the primary key is
equivalent to an exact subset.