The table space high water mark

The high water mark refers to the page number of the first page in the extent following the last allocated extent.

For example, if a table space has 1000 pages and an extent size of 10, there are 100 extents. If the 42nd extent is the highest allocated extent in the table space that means that the high-water mark is 420.
Tip: Extents are indexed from 0. So the high water mark is the last page of the highest allocated extent + 1.
Practically speaking, it's virtually impossible to determine the high water mark yourself; there are administrative views and table functions that you can use to determine where the current high water mark is, though it can change from moment to moment as row operations occur.

Note that the high water mark is not an indicator of the number of used pages because some of the extents below the high-water mark might have been freed as a result of deleting data. In this case, even through there might be free pages below it, the high water mark remains as highest allocated page in the table space.

You can lower the high water mark of a table space by consolidating extents through a table space size reduction operation.

Examples

Figure 1 shows a series of allocated extents in a table space.
Figure 1. High water mark
Illustration of the concept of the high water mark in a table space
When an object is dropped, space is freed in the table space. However, until any kind of storage consolidation operation is performed, the high water mark remains at the previous level. It might even move higher, depending how new extents to the container are added.