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Category: ASE: Data Storage

Page, Logical Page, Adaptive Server Page, Sybase Page

A page is the basic unit for data storage in ASE. The size of a page, which is maintained by ASE, is set during installation of ASE to 2K (prior to version 12.5), 4K, 8K or 16K betragen (starting with version 12.5.0). After successfully installing ASE Server the page size cannot be changed. The size of a page together with the size of the buffer cache determines the memory used and the i/o throughput of the server.

Pages store database objects. Depending on their usage, certain page types are differentiated.

Page Types

  • data pages – store data rows for tables.
  • index pages - store index rows for all types of indexes.
  • large object pages – (large object = LOB) LOB pages store data for columns defined with a text and image datatype as well as for all Java off-row columns.
  • Global allocation map pages or GAM pages - contain allocation bitmaps for databases.
  • Object allocation map pages or OAM pages - OAM pages contain information on used extents for an object. Every table or index contains at least one OAM page and tracks information on the location of pages for each object in the database.
  • Allocation pages - allocation pages contain information on space usage and objects in groups of 256 pages.
  • Control pages - Control pages govern the space usages for partitioned tables. Every partition is administered by an own control page.

Pages: Important Commands and Topics

The size of pages controls the storage utilisation in ASE. Every allocation page, object allocation map (OAM) page, data page, index page, LOB page is based on a logical page size defined when installing ASE. If ASE was installed with 16K page size, all pages types are 16K in size and consequently, all page types will take up 16KB of physical storage. Additionally, large OAM pages (in this case 16K OAM pages) contain more OAM entries than smaller (8K, 4K oder 2K) pages.

Page headers/overhead and page sizes

All pages contain a so-called header or overhead. The header stores information like "previous page pointer" in an object, "next page pointer" in an object, "page id Nummer", the "Object id" to which a page belongs, or data for storage maintenance of the page. The size of the header depends on the locking scheme:

  • The header of an APL 2K page ist 32 bytes large. This leaves 2016 bytes for storing data.
  • The header of a DOL 2K page is 44 bytes. In addition, the bottom end of a DOL-page contains a 2-byte timestamp, used for database consistency checks. This leaves 2002 bytes for storing data.

As the size of the page header does not vary for different page sizes, the relative amount of space available for storing data is larger for larger page sizes than for the standard 2K page size.