Re: locally managed autoallocate (was: Separate Indexes and Data)

Yes, and there is one thing to add:
If you do not specify INTIAL, the extent allocation starts with 5 blocks for the intial extent. For 8k, it's 40k, but in an autoallocating LMT extent cannot be smaller then 64k, so it is the amount of the space allocated. The interesting question is: what happens with blocksize-16k? Will there be 64k or two extents of 64k, i.e. 128k?
Here is the answer:

SQL> create tablespace test1
  2 datafile '/data/oradata/data/test101.dbf' size 64M reuse   3 autoextend on next 64m maxsize 513M   4 extent management local autoallocate   5 segment space management auto
  6 blocksize 16k
  7 /

Tablespace created.

SQL> create table a (a number) tablespace test1;

Table created.

SQL> select owner,segment_name,extent_id,blocks   2 from dba_extents
  3 where segment_name='A'and tablespace_name='TEST1'   4 and owner=user
  5 /

OWNER                          SEGMENT_NA  EXTENT_ID     BLOCKS
------------------------------ ---------- ---------- ----------
OPS$MGOGALA                    A                   0         64

16k*64=1M. That means that oracle will allocate a full megabyte for the initial extent. It cannot take 64k, because it's smaller then 5*16k (that number of 5 blocks is hardwired into the RDBMS since time immemorial) and it cannot take two extents because that would, in turn, mean that the initial extent is smaller then 5 blocks. Therefore, it takes 1M. Jonathan Lewis was right. Here is one tecnique for optimizing the disk consumption in such cases:

SQL> drop tablespace test1 including contents and datafiles;

Tablespace dropped.

SQL> On 2003.09.30 20:34, Jacques Kilchoer wrote:
> > Ive read the book. PCTINCREASE is basically set to 100% so
> > the extent sizes double. Thats 'basically' how it works. I
> > have seen some posts on dejanews saying it doesnt necessarily
> > work this way and some people are finding large extent sizes
> > with just a few extents and when tables are dropped this is
> > leading to fragmentation. It hasnt happened to me, but the
> > posts on dejanews were from some pretty good posters. So Im
> > playing conservative. We also had one of the contributors
> > here mention issues.
>
>
> I think Jonathan Lewis has explained the algorithm before, but it's also
> something that we have investigated here.
> The algorithm (ignoring some details) is:
> There will be 4 extent sizes used, 64K, 1M, 8M, 64M
> As long as object allocation is 1M or less, 64K extent sizes are used,
> When object allocation is between 1M and 64M, 1M extent sizes are used.
> When object allocation is between 64M and 1G, 8M extent sizes are used.
> When object allocation is more than 1G, 64M extent sizes are used.
>
> However, when you initially create the object, the extents are determined by
> figuring out the space allocated to the newly created object taking into
> account the INITIAL, NEXT, PCTINCREASE, MINEXTENTS storage parameters. So
> the
> object might start off with 1M extents instead of starting off with 64K
> extents. The algorithm is similar to the one outlined above but it is more
> complicated. The NEXT and PCTINCREASE seem to be ignored after the object is
> created.
> e.g.
> create table ... tablespace locally_managed_autoallocate
> storage (initial 1M next 512K minextents 15 pctincrease 0) ... ;
> Initial allocation will be 1M + (15 - 1) * 512K = 8M
> When you create the table, you will see eight extents, each of one megabyte.
>
> There are additional wrinkles, but I don't think the algorithm has "bugs".
>
> I don't think that there really is "fragmentation" in the sense that an
> unused extent will remain unused forever. All extents will be in one of the
> 4
> sizes mentioned above, and all are subject to reuse at some point.
> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Jacques Kilchoer
> INET: Jacques.Kilchoer_at_quest.com
>
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-- 
Mladen Gogala
Oracle DBA
-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: Mladen Gogala
  INET: mgogala_at_adelphia.net

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