Solved

Properly sizing the extents

Posted on 1998-04-12
4
693 Views
Last Modified: 2011-10-03
We have a Oracle database(7.3.2.1)  with db_block_size 8k on Sun UltraSparc machine(Solaris Ver 2.6).
I created a tablepsace with the default storage parameter  'initial extent'  as 64K(ie  8 db blocks). But the objects created in the tablespace takes up 80K(ie 10db blocks) for its initial extent. I created the object in the tablespace by explicitly specifying the storage parameter for the object.(with initial extent size as 64K). This also creates extents with 80K.
How to prevent the Oracle from doing this rounding and get the correct extent size which I need?
0
Comment
Question by:srikumar_p
  • 2
  • 2
4 Comments
 
LVL 2

Accepted Solution

by:
xiaodong earned 50 total points
Comment Utility
When Oracle allocate new extents, the smallest increment is 5 DB blocks, i.e., for a database using 2K block, the increment is 10K, 4K, it's 20K. Thus, in your case, it's 80K(2x5=10 DB blocks) instead of 64K as you specified.  
Specifying a bigger block size as VS. the default 2K size will yield better performance but at the expense of a little waste of space.  If you can live with that 16K 'waste', then do nothing, otherwise, recreate your database using a smaller block size.
If I were you, I will do nothing.  Since the 'waste' will not really be wasted in the long run.
0
 

Author Comment

by:srikumar_p
Comment Utility
Thanks xiaodong for the answer.
I don't mind wasting 16K. But the thing is I have set the db_multiblock_read_count parameter as 8. That means Oracle will read 64K in one read.(8*8Kb) If my extent size is 80K this will result in 2 reads(during full table scans)- one for the first 64K and the second one for remaining 16K. These reads can't span extents. So this will reduce the preformance as two reads are required for an extent. Is it advisable to set the db_multiblock_read_count to 10?

Then, I don't think the smallest increment is 5 DB blocks. Because I am able to create the extent with 4 DB blocks(ie 32KB in my database). But any thing above 4 it rounds to 10.
0
 
LVL 2

Expert Comment

by:xiaodong
Comment Utility
db_multiblock_read_count really depends on the file I/O on the O/S.  I believe most UNIX systems allows at most 64K read in one I/O operation.  So this parameter should always set to take advantage of the O/S capability.  In your case, 8 is fine.  If you set it to 10, you will see no gains at all.
Secondly, if you can show me any proof(such as select from dba_segments) that you are able to create an extent of 32K in a database using block size of 8K, I will be very surprised.
0
 

Author Comment

by:srikumar_p
Comment Utility
Hi xiaodong,
Thanks for your suggestions.
 For the 32K extent using 8K db block size - you can see the commands I cut and pasted from
sql*plus
SQL> create table emp (empno integer, empname varchar2(20))
  2  storage (initial 32k minextents 1);

Table created.

SQL> select * from user_extents where segment_name = 'EMP';

SEGMENT_NAME                                                                      SEGMENT_TYPE
--------------------------------------------------------------------------------- -----------------
TABLESPACE_NAME                EXTENT_ID     BYTES    BLOCKS
------------------------------ --------- --------- ---------
EMP                                                                               TABLE
DATA_TBSP                              0     32768         4


SQL>

and the db_block_size is 8K- trust me :-)

By the way, can I know more about you? Where are you working? What environment etc. etc.
My mail id is kumarp@wipsys.soft.net. You can mail to me directly rather than through the web.

The web is very slow during our office hours. It is World Wide Wait(www) really.

Thanks again.

0

Featured Post

PRTG Network Monitor: Intuitive Network Monitoring

Network Monitoring is essential to ensure that computer systems and network devices are running. Use PRTG to monitor LANs, servers, websites, applications and devices, bandwidth, virtual environments, remote systems, IoT, and many more. PRTG is easy to set up & use.

Join & Write a Comment

Truncate is a DDL Command where as Delete is a DML Command. Both will delete data from table, but what is the difference between these below statements truncate table <table_name> ?? delete from <table_name> ?? The first command cannot be …
Have you ever had to make fundamental changes to a table in Oracle, but haven't been able to get any downtime?  I'm talking things like: * Dropping columns * Shrinking allocated space * Removing chained blocks and restoring the PCTFREE * Re-or…
This video shows how to recover a database from a user managed backup
This video shows how to copy an entire tablespace from one database to another database using Transportable Tablespace functionality.

771 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

10 Experts available now in Live!

Get 1:1 Help Now