Because all extents are the same size, there can be no fragmentation.
Gone is the classic example of a 51MB segment that can??™t be allocated in a tablespace with two
free 50MB extents because the two 50MB extents are not adjacent.
On the other hand, automatic segment extent management within a locally managed tablespace
allocates space based on the size of the object. Initial extents are small, and if the object stays
small, very little space is wasted. If the table grows past the initial extent allocated for the segment,
subsequent extents to the segment are larger. Extents in an autoallocated LMT have sizes of 64KB,
1MB, 8MB, and 64MB, and the extent size increases as the size of the segment increases, up to a
maximum of 64MB. In other words, Oracle is specifying what the values of INITIAL, NEXT, and
PCTINCREASE are automatically, depending on how the object grows. Although it seems like
fragmentation can occur in a tablespace with autoallocation, in practice the fragmentation is
minimal because a new object with a 64KB initial segment size will fit nicely in a 1MB, 4MB,
8MB, or 64MB block preallocated for all other objects with an initial 64KB extent size.
Given an LMT with either automatically managed extents or uniform extents, the free space
within the segment itself can be AUTO or MANUAL. With AUTO segment space management,
a bitmap is used to indicate how much space is used in each block. The parameters PCTUSED,
FREELISTS, and FREELIST GROUPS no longer need to be specified when the segment is created.
Pages:
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322