Using ASSM reduces segment header contention and improves simultaneous insert concurrency;
this is because the free space map in a segment is spread out into a bitmap block within each
extent of the segment. As a result, you dramatically reduce waits because each process performing
insert, update, or delete operations will likely be accessing different blocks instead of one freelist
or one of a few freelist groups. In addition, each extent??™s bitmap block lists each block within the
extent along with a four-bit ???fullness??? indicator defined as follows (with room for future expansion
from values 6??“15):
0000 Unformatted block
0001 Block full
0010 Less than 25 percent free space available
0011 25 percent to 50 percent free space
0100 50 percent to 75 percent free space
0101 Greater than 75 percent free space
In a RAC database environment, using ASSM segments means you no longer need to create
multiple freelist groups. In addition, you no longer need to specify PCTUSED, FREELISTS, or
FREELIST GROUPS parameters when you create a table; if you specify any of these parameters,
they are ignored.
Extents
An extent is the next level of logical space allocation in a database; it is a specific number of
blocks allocated for a specific type of object, such as a table or index. An extent is the minimum
number of blocks allocated at one time; when the space in an extent is full, another extent is
allocated.
When a table is created, an initial extent is allocated.
Pages:
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310