The database server allocates the pages that belong to a tblspace as extents. Although the pages within an extent are contiguous, extents might be scattered throughout the dbspace where the table resides (even on different chunks).
Figure 49 depicts this situation with two noncontiguous extents that belong to the tblspace for table_1 and a third extent that belongs to the tblspace for table_2. A table_2 extent is positioned between the first table_1 extent and the second table_1 extent. When this situation occurs, the extents are interleaved. Because sequential access searches across table_1 require the disk head to seek across the table_2 extent, performance is worse than if the table_1 extents were contiguous. For instructions on how to avoid and eliminate interleaving extents, see your IBM Informix Dynamic Server Performance Guide.