Use the ALTER DATAFS option when you want to enable the DATA FAST SEARCH feature.
Prior to Version 12.0, each data area that contained a single table that was not compressed was searched for a record using a binary search. The ability to do the binary seach was turned on at data load time if a physical block could contain 15 or more data rows. Otherwise, the blocks were searched in a sequential physical order. For large block sizes with small records, such a sequential search could become a significant percentage of MUF CPU usage. Using DATAFS, however, the last 16 bytes of each data block is used by the search algorithm to provide a faster method of finding individual data rows, especially for large block and small row sizes. Once created, these 16 end bytes are stored on DASD with the data.
Migration Considerations
Because using the feature that reduces compression overhead physically alters the data area, there are two fallback considerations to consider, that is, you must consider what happens if:
In both cases, reading and searching the data block is not affected because each data block has stored within it the available space in the block. When searching the block, that information defines when the logical end of the block is reached. The available space includes the 16 bytes used by the DATAFS search algorithm. It is therefore completely transparent to the old block search code. Even if you fall back to the previous release, unless maintenance is done to a row within the block the 16 bytes remain at the end of the block.
If a data block is changed due to a maintenance command after the option has been turned off, the maintenance clears the last 16 bytes, if it has been used by DATAFS. A previous release fallback solution that does the same thing is provided.
The remaining concern for the previous release is to consider what would happen if the fallback solution has not been implemented at a site which falls back after setting the global DATAFS option.
As each data area data set is opened, the DATAFS option is checked. If it is YES and it has not already been done for this data area, it is marked as incompatible with the previous release (unless the fallback solution is in place). Future previous release opens of this data set fail with a return code 72 (003) if the data area has been thus marked and if the previous release fallback solution is not in place.
The DBUTLTY INIT of a data area also marks the data area, no matter the DATAFS setting.
Reporting DATAFS in Full CXX Report
A full CXX report includes information on DATA FAST SEARCH status, where status in the report is either YES or NO.
Security Considerations
This functions is secured in the DTUTIL resource class with a resource name ending in CXXMAINT.ALTEROP2. See the DBUTLTY Resource List section of the CA Datacom Security Reference Guide for details. No underlying table rights are checked.
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