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Oracle AWR I/O Reports Section

Oracle Tips by Burleson Consulting

AWR Report

Disk I/O Reports Section

The following sections of the AWR report show the distribution of I/O activity between the tablespaces and data files.


Tablespace IO Stats  DB/Inst: LSQ/lsq  Snaps: 1355-1356

-> ordered by IOs (Reads + Writes) desc




                 Av      Av     Av                       Av     Buffer Av Buf

         Reads Reads/s Rd(ms) Blks/Rd  Writes Writes/s      Waits Wt(ms)

-------------- ------- ------ ------- ------- - ---------- ------


        26,052       7  132.5    14.5       1 0     43,931   78.3


         1,730       0  123.5     1.1   1,139 0          0    0.0


           814       0  305.1     2.0      95 0          0    0.0


           262       0   15.2     1.0       1 0        290   12.1


            14       0   86.4     1.0     129 0          3    0.0


             1       0   80.0     1.0       0 0          0    0.0



File IO Stats  DB/Inst: LSQ/lsq  Snaps: 1355-1356

-> ordered by Tablespace, File


Tablespace               Filename

------------------------ ----------------------------------------------------

                 Av      Av     Av                       Av  Buffer Av Buf

         Reads Reads/s Rd(ms) Blks/Rd       Writes Writes/s  Waits Wt(ms)

-------------- - ------ ------- ------------ -------- --- ------


         1,730 0  123.5     1.1        1,139        0  0    0.0


           814 0  305.1     2.0           95        0  0    0.0


             1 0   80.0     1.0            0        0  0


        26,052 7  132.5    14.5            1        0  43,931   78.3


            14 0   86.4     1.0          129        0  3    0.0


           262 0   15.2     1.0            1        0  290   12.1



In general, the information presented in the sample AWR I/O section shown above is intended to help the DBA identify hot spots of the database I/O subsystem.


Oracle considers average disk read times of greater than 20 milliseconds to be unacceptable. If data files, as in the example above, consistently have average read times of 20 ms or greater, a number of possible approaches can be followed:

SQL Management: A database with no user SQL being run generates little or no I/O. Ultimately all I/O generated by a database is directly or indirectly due to the nature and amount of user SQL being submitted for execution. This means that it is possible to limit the I/O requirements of a database by controlling the amount of I/O generated by individual SQL statements. This is accomplished by tuning SQL statements so that their execution plans result in a minimum number of I/O operations. Typically in a problematic situation, there will only be a few SQL statements with suboptimal execution plans generating a lot more physical I/O than necessary and degrading the overall performance for the database.

Using Memory Caching to Limit I/O: The amount of I/O required by the database is limited by the use of a number of memory caches; e.g., the Buffer Cache, the Log Buffer, various Sort Areas etc. Increasing the Buffer Cache, up to a point, results in more buffer accesses by database processes (logical I/Os) being satisfied from memory instead of having to go to disk (physical I/Os). With larger Sort Areas in memory, the likelihood of them being exhausted during a sorting operation and having to use a temporary tablespace on disk is reduced.

Tuning the Size of Multi-Block I/O: The size of individual multi-block I/O operations can be controlled by instance parameters. Up to a limit, multi-block I/Os are executed faster when there are fewer larger I/Os than when there are many smaller I/Os.

Index Management: If the tablespace contains indexes, another option is to compress the indexes so that they require less space and hence, less I/O.

Optimizing I/O at the Operating System Level: This involves making use of I/O capabilities such as Asynchronous I/O or using File systems with advanced capabilities such as Direct I/O, bypassing the Operating Systemís File Caches. Another possible action is to raise the limit of maximum I/O size per transfer.

Load Balancing: Balancing the database I/O by use of Striping, RAID, Storage Area Networks (SAN) or Network Attached Storage (NAS). This approach relies on storage technologies such as Striping, RAID, SAN and NAS to automatically load balance database I/O across multiple available physical disks in order to avoid disk contention and I/O bottlenecks when there is still available unused disk throughput in the storage hardware.

I/O Management: Database I/O by manual placement of database files across different file systems, controllers and physical devices. This is an approach used in the absence of advanced modern storage technologies. Again, the aim is to distribute the database I/O so that no single set of disks or controller becomes saturated from I/O requests when there is still unused disk throughput. It is harder to get right than the previous approach and most often less successful.

Volume: Reducing the data volumes of the current database by moving older data out.

Hardware: Investing in more modern and faster hardware.



This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts:




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