Structure of a Blobspace

When you create a blobspace, you can specify the effective size of the data pages, which are called blobpages. The blobpage size for the blobspace is specified when the blobspace is created. Blobpage size must be a multiple of page size. (For information on determining database server page size, see the chapter on managing disk space in the IBM® Informix® Administrator's Guide.) All blobpages within a blobspace are the same size, but the size of the blobpage can vary between blobspaces. Blobpage size can be greater than the page size because data stored in a blobspace is never written to the page-sized buffers in shared memory.

The advantage of customizing the blobpage size is storage efficiency. Within a blobspace, TEXT and BYTE data is stored in one or more blobpages, but simple large objects do not share blobpages. Storage is most efficient when the TEXT or BYTE data is equal to or slightly smaller than the blobpage size.

The blobspace free-map pages and bitmap pages are the size specified as a database server page, which enables them to be read into shared memory and to be logged.

When the blobspace is first created, it contains the following structures:
  • Blobspace free-map pages
  • The blobspace bitmap that tracks the free-map pages
  • Unused blobpages

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