The records in a data set are normally blocked. Users can control the size of a block using the BLOCKSIZE option. Block size can be specified in one of the following options:
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WORDS
The maximum value is 65520 words.
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SEGMENTS
The maximum value is 2184 segments. This is equivalent to 65520 words rounded down to segments.
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BYTES
The maximum value is 65535 bytes.
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RECORDS
This is the default unit. The maximum value is 65520 records. This can be used to define a block size larger than can be defined by the WORDS, SEGMENTS or BYTES option.
| Note: | For recommendations about block size values in anticipation of the deployment of 512-byte physical sector technology, refer to “Preparing for Open Disk Technology” at the beginning of this section. |
Every block occupies an integral number of segments on disk. If the number of records for each block does not use an integral number of segments, then the remainder of the last segment of each block is wasted.
When the block size is specified in records, the specified value is used. When the block size is specified in units other than records, the block size is rounded up to the nearest unit.
When the block size is specified in words, bytes, or segments, the actual block size is less than or equal to the specified value. The specified block size value is used to determine the number of records that can fit in a block after allowing for the control words. The actual block size is the record size in words multiplied by the number of records per block plus the control words, rounded up to the nearest specified unit. The number of control words reserved in each block depends on the data set options, such as CHECKSUM and ADDRESSCHECK, and the data set type.
When no block size is specified, DASDL automatically computes a default value, taking in consideration the value of ADDRESSCHECK, CHECKSUM, and other control words. For embedded data sets, the system attempts to make the block size equal the population. The length of the fixed portion of the record is used to calculate the block size for variable-format records.
Be sure to take into account the explicit or implicit setting of control words when calculating data set BLOCKSIZE specifications.
