Blobs

Blobs held in a blobspace do not use the buffer pool or the logical log. To facilitate rollback and data integrity the engine maintains two copies of a blob once it has been updated, a before and an after image. The disk associated with the before image will not be released until the logical log containing the transaction record has been written to tape (or disk). Therefore, if the blobspace have a significant number of updates applied then the disk space required can be much larger than first anticpdated.

Always use a blobspace. By avoiding the standard table spaces, it bypasses the logical log and buffer cache. Large blobs in tablespaces will decrease the the overall engine cache rate.

The blob page size should be large enough to hold 97% of all the blob rows (i.e. the mean blob size plus one standard deviation). If a blob can be saved in a single blob page then it will be save in a contiguous area on the disk making retrieval a single disk read. If the blob doesn't fit in a single page then several reads will be required to retrieve the blob from several pages, probably scattered across the disk. A typical A4 document saved in compressed TIFF format at a standard printer resolution will require 40K per page, a screen version is 20K.

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