TAILIEUCHUNG - OCA /OCP Oracle Database 11g A ll-in-One Exam Guide- P25

OCA /OCP Oracle Database 11g A ll-in-One Exam Guide- P25:There is an ever increasing demand for staff with IT industry certification. The benefits to employers are significant—they can be certain that staff have a certain level of competence—and the benefits to the individuals, in terms of demand for their services, are equally great. Many employers are now requiring technical staff to have certifications, and many IT purchasers will not buy from firms that do not have certified staff. | OCA OCP Oracle Database 11g All-in-One Exam Guide 196 7T TIP Converting tablespaces to local management is quick and easy except for the SYSTEM tablespace where some extra steps are required. These are hfflBfflS well documented in the System Administrator s guide available as part of the product documentation. Segment Space Management The segment space management method is set per tablespace and applies to all segments in the tablespace. There are two techniques for managing segment space usage manual or automatic. The difference is clear automatic management should always be used manual management should never be used. Manual segment space management is still supported but never recommended. Like dictionary-managed extent management it is a holdover from previous releases. Automatic segment space management was introduced with release 9i and has become the default with release 11g. Every segment created in an automatic management tablespace has a set of bitmaps that describe how full each block is. There are five bitmaps for each segment and each block will appear on exactly one bitmap. The bitmaps track the space used in bands or ranges there is a bitmap for full blocks and there are bitmaps for blocks that are 75 percent to 100 percent used 50 percent to 75 percent used 25 percent to 50 percent used and 0 percent to 25 percent used. When searching for a block into which to insert a row the session server process will look at the size of the row to determine which bitmap to search. For instance if the block size is 4KB and the row to be inserted is 1500 bytes an appropriate block will be found by searching the 25 percent to 50 percent bitmap. Every block on this bitmap is guaranteed to have at least 2KB of free space. As rows are inserted deleted or change size through updates the bitmaps get updated accordingly. The old manual space management method used a simple list known as the free list which listed which blocks were available for inserts but without any .

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