SQL/MX 2.x Reference Manual (G06.24+, H06.03+)

SQL/MX File Attributes
HP NonStop SQL/MX Reference Manual—523725-004
8-6
EXTENT
EXTENT
EXTENT is a Guardian file attribute that sets the size of the extents (units of
contiguous disk space) that will be allocated for a file or partition of a file. EXTENT
applies to tables and indexes and is set when a file or partition is created.
The default is 16 pages for the primary extent and 64 pages for each secondary extent.
A page consists of 2048 bytes.
integer
is an integer that specifies the number of pages in the extent. The ranges allowed
are from 0 pages to the number of pages that will fit on a disk. The only limit is the
physical amount of storage available.
Each partition of a partitioned file has its own EXTENT attribute that can differ from
the EXTENT attribute for other partitions of the file. You can specify a single
EXTENT size for each extent in the file or partition, or you can specify one size for
the primary (first) extent and another size for the secondary extents.
If you enter only one value, with or without parentheses, it will be used for both the
primary and secondary extent sizes. If you enter two values, they will be used for
primary and secondary extent sizes.
Considerations for 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, NonStop SQL/MX uses the next block size
or the next full page size. For example, 0 PAGE rounds up to 2 PAGEs.
A file (or 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
EXTENT ext-size | (pri-ext-size [, sec-ext-size ])
ext-size, pri-ext-size, sec-ext-size is:
integer [PAGE[S] ]