COMPACTB command

Summary

Privileges: File Manager
Function: Combine as many extension records as possible into one extension record for the current or specified file.

Syntax

COMPACTB [FROM ssss][TO eeee][FREE nn][MAXE nn] [DELETE]

Where

FROM ssss indicates the starting Table B page number where the compactor starts looking for extensions. The default is zero.

TO eeee indicates the ending logical record number where the compactor stops working.

FREE nn indicates the percentage of unused or free pages (Table B or Table X) that may be used by the data compactor for new extension records. The default is 10 percent. The percent of free pages is calculated as follows:

Table B: ((BSIZE - BHIGHPG) / BSIZE) * 100
Table X ((XSIZE - XHIGHPG / XSIZE) * 100

MAXE nn specifies the percentage of a page size (6144 bytes) that defines the maximum extension record size eligible for compaction. Larger extensions are not moved. The default is 80 percent.

DELETE specifies to physically delete logically deleted records. The default behavior is to not delete the logically deleted records.

Usage

The data compactor is a CPU and I/O intensive process; Rocket recommends that you do not compact data at your site during high load periods. To avoid monopolizing system resources, the data compactor checks for bump or long request conditions at the following intervals:

When compacting files with Table X, the check for bump or long request conditions is done for every:
- 30 compacted records, or
- 30 processed Table B pages, or
- 30 logically deleted records,
whichever comes first.
When compacting files with no Table X, COMPACTB allows higher priority users to run at every 30 records read, regardless of whether the records are compacted or not.

The data compactor commits at every record compacted.

If your system crashes, only one record compaction may be lost. Data is never lost. The worst that may happen when the system crashes is that after recovery processing, one record-and only one-will have either the:

Old extension chain with the new extension chain being orphaned, or
New extension chain with the old extension chain being orphaned

You cannot reclaim space for orphaned chains without a file reorganization. There is no back out for the data compactor.

The compactor takes preimages of all the pages it will change and writes a journal record, containing all compacted extensions for each compaction, which may require that you increase the checkpoint and journal data set sizes.

There is no restriction on record length or number of extension records for the data compactor, as long as there is enough free space on pages to hold the compacted records.

The COMPACTB command maintains the same logical order-that is visible to programs-of extension records in the compacted records as in the original record. Their physical order-that is, the order of the page numbers-may differ.

Setting the DELETE argument

To use the DELETE argument of the COMPACTB command for a file, that file must already have a Table X defined for it-XSIZE is greater than zero in the file CREATE arguments.

Setting the MAXE argument

The larger an extension, the less likely that it will be combined with other extensions, because the largest single extension record may not be larger than a page size. When extension chains are very long and contain mainly very short extensions, a smaller MAXE setting may produce better results more quickly. You might want to test various settings to find what percent is most advantageous for your data.

Running the data compactor

The data compactor tries to lock records on a one-at-a-time basis. Records with extensions that are subject to compaction are locked exclusively as long as compaction for the record takes place. If record lock is not available, then the record is skipped. Avoid running the data compactor with applications that lock large numbers of records for a long time.

Reviewing data compactor statistics

At the end of processing, the compactor prints the following statistics:

M204.2749: NUMBER OF BASIC RECORDS PROCESSED: nnnn M204.2750: NUMBER OF EXTENSION RECORDS BEFORE COMPACRTION: nnnn M204.2751: NUMBER OF EXTENSION RECORDS AFTER COMPACTION: nnnn M204.2752: NUMBER OF NOT PROCESSED (LOCKED) RECORDS: nnnn M204.2753: NUMBER OF FREE PAGES USED: nnnn M204.2754: NUMBER OF DELETED LOCIGALLY DELETED RECORDS: nnnn

Example: file statistic changes

>IN TEST 1 VIEW BHIGHPG,EXTNADD,EXTNDEL,BQLEN BHIGHPG 39 TABLE B HIGHEST ACTIVE PAGE EXTNADD 42 EXTENSION RECORDS ADDED EXTNDEL 15 EXTENSION RECORDS DELETED BQLEN 2 TABLE B QUEUE LENGTH IN TEST1 COMPACTB FROM 0 TO 9999999 FREE 100 MAXE 100 NUMBER OF BASIC RECORDS PROCESSED: 20 NUMBER OF EXTENSION RECORDS BEFORE COMPACTION: 27 NUMBER OF EXTENSION RECORDS AFTER COMPACTION: 20 NUMBER OF NOT PROCESSED (LOCKED) RECORDS: 0 NUMBER OF FREE PAGES USED: 2 IN TEST1 VIEW BHIGHPG,EXTNADD,EXTNDEL,BQLEN BHIGHPG 41 TABLE B HIGHEST ACTIVE PAGE EXTNADD 49 EXTENSION RECORDS ADDED EXTNDEL 29 EXTENSION RECORDS DELETED BQLEN 5 TABLE B QUEUE LENGTH

At the end of processing, the compactor prints the following statistics:

M204.2749: NUMBER OF BASIC RECORDS PROCESSED: nnnn M204.2750: NUMBER OF EXTENSION RECORDS BEFORE COMPACTION: nnnn M204.2751: NUMBER OF EXTENSION RECORDS AFTER COMPACTION: nnnn M204.2752: NUMBER OF NOT PROCESSED (LOCKED) RECORDS: nnnn M204.2753: NUMBER OF FREE PAGES USED: nnnn M204.2754: NUMBER OF DELETED LOCIGALLY DELETED RECORDS: nnnn

The value of BQLEN parameter also changed. When the original extensions were deleted, a page became suitable for reuse and was placed on the Reuse Queue. In this example unused pages were used for new extensions, if new extensions are placed on a page from the Reuse Queue, BQLEN may decrease.

Example: compacting extensions

A single record has nine extension records with the following length. The lengths of the extensions in this example were chosen arbitrarily to illustrate how and when extensions are combined, or not combined.

`Ext.1`	Ext.2	Ext.3	Ext.4	Ext.5	Ext.6	Ext.7	Ext.8	Ext.9
`40`	1200	2400	3200	4300	2300	60	90	120

The chain is reduced to four parts:

Part 1: Extensions 1,2, and 3 have a combined length of 3640 that compacts into one extension record.

Part 2: Extension 4 is not moved, because combined with previous extensions it will not fit the page.

Part 3: Extension 5 is not moved, because it cannot be combined with either Extension 4 or Extension 6.

Part 4: Extensions 6,7,8, and 9 have combined length of 2570 that compacts into one extension record.

After compacting, there are four extensions instead of nine.