Block size

All I/O occurs in units of blocks. The size of these blocks is set when you create a Sybase 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 128KB results in a default block size of 8192 bytes. In general, Sybase 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:

Table 4-3 shows the default block size for each IQ page size.

Table 4-3: Default block sizes

IQ page size (KB)

Default block size (bytes)

64

4096

128 (default for new databases)

8192

256

16384

512

32768