IBM 000-8697 User Manual

System Architecture

Data rows that include blob data do not include the blob data in the row itself.
Instead, the data row contains a 56-byte blob descriptor that includes a
forward pointer (rowid) to the location where the first segment of blob data
is stored. The descriptor can point to a blob page (if the blob is stored in a
dbspace) or a blobpage (if the blob is stored in a blobspace).

Following is the structure of the 56-byte blob descriptor:

typedef struct tblob
    {
    short

tb_fd;

/* blob file descriptor (must be first) */

    short

tb_coloff;

/* Blob column offset in row

*/

    long

tb_tblspace;

/* blob table space*/

    long

tb_start;

/* starting byte*/

    long

tb_end;

/* ending byte: 0 for end of blob */

    long

tb_size;

/* Size of blob

*/

    long

tb_addr;

/* Starting Sector or BlobPage

*/

    long

tb_family;

/* Family Number (optical support)*/

    long

tb_volume;

/* Family Volume

*/

    short

tb_medium;

/* Medium - one if optical */

    short

tb_bstamp;

/* first BlobPage Blob stamp

*/

    short

tb_sockid;

/* socket id of remote blob*/

    short

tb_flags;

/* flags */

    long

tb_sysid;

/* optical system identifier*/

    long

tb_reserved2;

/* reserved for the future*/

    long

tb_reserved3;

/* reserved for the future*/

    long

tb_reserved4;

/* reserved for the future*/

    } tblob_t;

When a row containing blob data is to be inserted, the blobs are created first.
After the blobs are written to disk, the row is updated with the blob
descriptor and inserted.

0110

Blob page, room for partial-page segments

1010

Blob page, room for small segments

1110

Blob page, no room for even small segments

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