Sybase 12.4.2 manual Block size, 430

Overview of memory use

Because the Catalog Store accounts for only a tiny fraction of I/O, the page size for the Catalog Store has no real impact on performance. The default value of 4096 bytes should be adequate.

The IQ page size determines two other performance factors, the default I/O transfer block size, and the maximum data compression for your database. These factors are discussed in the sections that follow.

Block size

All I/O occurs in units of blocks. The size of these blocks is set when you create an IQ database; you cannot change it without recreating the database. By default, the IQ page size determines the I/O transfer block size. For example, the default IQ page size of 64KB results in a default block size of 4096 bytes. In general, IQ uses this ratio of default block size to page size, but it considers other factors also.

The default block size should result in an optimal balance of I/O transfer rate and disk space usage for most systems. It does favor saving space over performance, however. If the default block size does not work well for you, you can set it to any power of two between 4096 and 32,768, subject to the constraints that there can be no fewer than two and no more than 16 blocks in a page. You may want to set the block size explicitly in certain cases:

•For a raw disk installation that uses a disk array, larger blocks may give better performance at the expense of disk space.

•For a file system installation, to optimize performance over disk space, the IQ block size should be greater than or equal to the operating system's native block size, if there is one. You may get better I/O rates if your IQ block size matches your file system’s block size.

Table 12-3 shows the default block size for each IQ page size.

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