Oracle tablespace tuning tips Oracle Database Tips by Donald Burleson

There are some general tip for ensuring optimal tablespace allocation and tuning of tablespaces:

• Always use "autoextend on":  This prevents a tablespace from ?hanging? in a condition where an object cannot extend.  When combined with ?maxextents unlimited? for the tables/indexes, the space management monitoring shifts from the tablespace to the disk mount point.
  
  
•  Always use ASM for the tablespace data files:  Oracle ASM's use of RAID 0+1 (striping and mirroring) ensures that you never see any chronic disk hot spots.

Many professional have questions about Oracle data file and tablespace management, and I've tried to start this FAQ with common Oracle data file questions.

>> Does the amount of free space inside the tablespace containing all the tables inside an OLTP Production database affect its performance?

Not if your SQL uses indexes. However, if have free space within a table's or indexes extents and you see your SQL doing full-scans, then yes, it might affect performance. I wrote "Oracle Tuning: The Definitive Reference" with details on all of this and scripts that you can run to see if you have a space-related performance problem.

>> Should there be a threshold for free space on the disk where the datafiles are located?

No. If you restrict their growth, the database will lock-up Oracle, as the database freezes when a table runs out of extents or waiting for a tablespace to extend.  Always use "maxextents unlimited" and "autoextend on".

>> If my tablespaces are on AUTOEXTEND mode, is it possible to find how much space they would swell on at the worst peak?

Mike Ault has Oracle predictive modeling spreadsheets for predicting tablespace growth.

>> How can I tell if my UNDO tablespace is large enough?

If you run out of UNDO you will get the message "ORA-30036: unable to extend segment by 8192 in undo tablespace 'UNDOTBS'" and the update will abort.  The size of your UNDO is governed by the total space required to store the before images for your largest DML transaction.  The size of your UNDO is governed by the total space required to store the before images for your largest DML transaction.

Some DBA's use the "autoextend" option on their UNDO tablespace, but you run the risk of running out of disk space if you have a super-large update running, or a runaway update. Like everything it Oracle, the choice us using qutoextend for UNDO depends on your system requirements. If your SLA requires you to minimize aborts, it might be worth the risk of filling-up a disk.

>> Is it better to increase the Oracle datafile size or add another datafile to increase the Oracle tablespace size?

Some DBA's choose to add datafiles instead of using autoextend for the tablespace.  The choice of adding datafiles depends on your RAID level and how many disk spindles you have. Mike Ault has an eBook dedicate to Oracle space management, and his book "Oracle Disk I/O Tuning" is superb.

Also see my collected notes on Oracle data file and tablespace management: