When a table includes a text, image, or Java off-row datatype, the table itself stores a pointer to the data value. The actual data is stored on a separate linked list of pages called a LOB (large object) chain.
Writing or reading a LOB value requires at least two disk accesses, one to read or write the pointer and one for subsequent reads or writes for the data. If your application frequently reads or writes these values, you can improve performance by placing the LOB chain on a separate physical device. Isolate LOB chains on disks that are not busy with other application-related table or index access.
When you create a table with LOB columns, Adaptive Server creates a row in sysindexes for the object that stores the LOB data. The value in the name column is the table name prefixed with a “t”; the indid is always 255. Note that if you have multiple LOB columns in a single table, there is only one object used to store the data. By default, this object is placed on the same segment as the table.
You can use sp_placeobject to move all future allocations for the LOB columns to a separate segment.
See the System Administraton Guide for more information.
