System Architecture 2-91

Structure of a Blobspace

Structure of a Blobspace
After disk initialization, you can create blobspaces.
When you create a blobspace, you can specify the effective size of the blob-
holding pages, calledblobpages. The blobpage size for the blobspace is
specified when the blobspace is created as a multiple ofBUFFSIZE (the page
size). All blobpages within a blobspace are the same size, but the size of the
blobpagecan vary between blobspaces. Blobpage size can be greater than the
pagesize because blob data stored in a blobspace is never written to the page-
sized buffers in shared memory.
Theadvantage of customizing the blobpage size is storage efficiency. Within
ablobspace, blobs are stored in one or more blobpages but blobs do not share
blobpages.Blob storage is most efficient when the blob is equal to, or slightly
smaller than, the blobpage size.
The blobspace free-map pages and bit-map pages are the size specified as
BUFFSIZE, 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 (page2-147)
■The bit map that tracks the free-map pages (page2-147)
■Unused blobpages