How Adaptive Server allocates memory

All database object pages are sized in terms of the logical page size, which you specify when you build a new master device. All databases—and all objects in every database—use the same logical page size. The size of Adaptive Server logical pages (2, 4, 8, or 16K) determines the server’s space allocation. Each allocation page, object allocation map (OAM) page, data page, index page, text page, and so on are built on a logical page. For example, if the logical page size of Adaptive Server is 8K, each of these page types are 8K in size. All of these pages consume the entire size specified by the size of the logical page. Larger logical pages allow you to create larger rows, which can improve your performance because Adaptive Server accesses more data each time it reads a page. For example, a 16K page can hold 8 times the amount of data as a 2K page, an 8K page holds 4 times as much data as a 2K page, and so on, for all the sizes for logical pages.

The logical page size is a server-wide setting; you cannot have databases that have various sizes of logical pages within the same server. All tables are appropriately sized so that the row size is no greater than the current page size of the server. That is, rows cannot span multiple pages.

Regardless of the logical page size for which it is configured, Adaptive Server allocates space for objects (tables, indexes, text page chains) in extents, each of which is eight logical pages. That is, if a server is configured for 2K logical pages, it allocates one extent, 16K, for each of these objects; if a server is configured for 16K logical pages, it allocates one extent, 128K, for each of these objects.

This is also true for system tables. If your server has many small tables, space consumption can be quite large if the server uses larger logical pages. For example, for a server configured for 2K logical pages, systypes—with approximately 31 short rows, a clustered and a nonclustered index—reserves 3 extents, or 48K of memory. If you migrate the server to use 8K pages, the space reserved for systypes is still 3 extents, 192K of memory. For a server configured for 16K, systypes requires 384K of disk space. For small tables, the space unused in the last extent can become significant on servers using larger logical page sizes.