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Changes the default block size used for new .MDX files.

Syntax

SET IBLOCK TO <expN>

<expN>

A number from 1 to 63 that sets the size of index blocks allocated to new .MDX files. The default value is 1. (The actual size in bytes is the number you specify multiplied by 512 bytes; however, the minimum size of a block is 1024 bytes.) To change the default, update the IBLOCK setting in PLUS.ini. To do so, either use the SET command to specify the setting interactively, or enter the IBLOCK parameter directly in PLUS.ini.

Description

Use SET IBLOCK to change the size of blocks in which dBASE Plus stores .MDX files on disk to improve the performance and efficiency of indexes. You can specify a block size from 1024 bytes to approximately 32K. The IBLOCK setting overrides any previous block size defined by the SET BLOCKSIZE command or specified in the PLUS.ini file. After the block size has been changed, new .MDX index files are created with the new block size.

Multiple index (.MDX) files are composed of individual index blocks (or nodes). Nodes contain the value of keys corresponding to individual records and provide the information to locate the appropriate record for each key value. Since the IBLOCK setting determines the size of nodes, the setting also determines the number of key values that can fit into each node. When a single node can't contain all the key values in an index, dBASE Plus creates one or more parent nodes. These intermediate nodes also contain key values. Instead of pointing to record numbers, however, intermediate nodes point to leaf nodes or other lower-level intermediate nodes. If you increase the size of index blocks and create a new .MDX file, the new and larger leaf nodes contain more key values.

Whether you can improve performance by storing key values in larger or smaller nodes depends on several factors: the distribution of data, if tables are linked together, the length of key values and the type of operation requested. Typically, every .MDX file contains more than one index tag. Finding the best setting for a given .MDX file requires experimentation because the best size for one index tag might not be the best size for another.

The following is a list of basic principles governing index performance.

Since nodes might not be sequential, dBASE Plus reads only one node at a time from the disk. Reading more than one node is usually inefficient, because typically the second node is not the next node in the sequential list.

Once a node is read into memory, dBASE Plus attempts to store it there for later use.

When users link several tables together, for example, with SET RELATION, performance is better if all the relevant nodes for the tables are in memory simultaneously. For example, if a large node for table B pushes out the previously read node for table A, dBASE Plus must find and read the table A node again from disk when the node for table A needs to be used again. If both nodes remain in memory, performance can be improved.

When tables have many identical key values, dBASE Plus might have to store them in many nodes. In this situation, performance might be improved by increasing the node size so that dBASE Plus reads fewer nodes from disk to load the same number of key values into memory.

Small node sizes can cause performance degradation. This occurs because as nodes are read in and out, dBASE Plus attempts to cache them all. When the small nodes are removed from memory by more recently read nodes, they leave unused spaces in memory that are too small to contain larger nodes. Over time, memory can become fragmented, resulting in slower performance.