SQL/MP Reference Manual

HP NonStop SQL/MP Reference Manual—523352-013

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Considerations—EXTENT



A file's extent size must be at least as large as its block size and must be a multiple

of the block size and a multiple of page size (2048 bytes). If you specify extent

sizes that do not meet these conditions, SQL uses the next block size or the next

full page size. For example, 0 PAGE rounds up to 1 PAGE.

A file (or a partition of a partitioned file) must fit on a disk, so the size of the primary

extent plus the total size of all secondary extents must not exceed the disk size.



A primary extent should be large enough to hold the file at the initial load, and

secondary extents should be large enough to accommodate growth. The faster the

growth, the larger the secondary extents should be.

To ensure adequate space for your file, choose extent sizes and a MAXEXTENTS

value large enough to accommodate the amount of data you expect to store in the

file.

Using large extents can improve performance by reducing the number of seeks.

The disadvantage of large extents is that an entire extent is allocated

simultaneously, leaving allocated but unused space on the disk while the extent

contains only a small amount of data. You can maximize the use of disk space by

specifying smaller extent sizes if performance is not an issue.