Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 8 - RAC Administration

Manual Space Management

In the manual method, free space within a segment is tracked by structures such as free lists and free list groups.

Within the first block of each segment, there is a segment header. The segment header contains a list of blocks with free space, called the master free list. Blocks are removed from the master free list when the free space in those blocks becomes less than that specified by the pctfree storage parameter. Blocks are added back to the master free list when their used space falls below the threshold specified by the pctused storage parameter.

The master free list is accessed and used by all the processes that insert or update an object. These processes remove and add blocks from free lists based on the space remaining after DML operations on those blocks. Simultaneous update to the free list involves writes to the segment header, and also may result in contention as shown in the Fig. 8.3.

Figure 8.3 Free List in the Segment Header