AWR – Profiling Database I/O

June 2, 2015December 29, 2019 ~ rajeevramdas@aol.com ~ Leave a comment

Oracle Awr (Automatic Workload Repository) statistics, captures and stores fine grained information about file reads and writes (aka i/o), that the database performed, during the course of execution of, application generated database workloads. When analyzing the read and write patterns of the database, it helps a lot to understand what type of activity is generating the reads and writes. With this stored information we can get an indepth understanding of the distribution of random and sequential reads and writes.

I use this information for getting a better understanding of the I/O profile, for my Exadata sizing exercises.

This information can be used to understand clearly how much of the i/o is from Temp activity, Datafile reads and writes, Archivelog writes, log writes, and whether these are small or large reads and writes.

To the best of my understanding the small reads and writes are those < 128k and the large reads and writes are those > 128k.

This information is contained mainly in two awr Views.

Dba_Hist_Iostat_FileType
Dba_Hist_Iostat_Function

Dba_Hist_Iostat_FileType

This view displays the historical i/o statistics by file type. The main filetypes are the following

Archive Log
Archive Log Backup
Control File
Data File
Data File Backup
Data File Copy
Data File Incremental Backup
Data Pump Dump File
Flashback Log
Log File
Other
Temp File

Dba_Hist_Iostat_Function

This view displays the historical i/o statistics by i/o function. The main i/o functions are the following

Recovery
Buffer Cache Reads
Others
RMAN
Streams AQ
Smart Scan
Data Pump
XDB
Direct Writes
DBWR
LGWR
Direct Reads
Archive Manager
ARCH

From everything i have seen sofar, these reads and writes can be directly co-related to the “Physical read total IO requests” and “Physical write total IO requests” system level statistics.

I have written a script that displays information from the above mentioned views and gives a detailed breakdown of i/o gendrated from different aspects of the database activities.
In order to fit in the computer screen real estate, i have actually limited the columns the script displays (So it displays only the file types i am frequently interested in). Please feel free to take the script and modify it to add columns that you want to display.

The full version of the script awrioftallpct-pub.sql can be found here.

The script accepts the following inputs
– A begin snap id for a snapid range you want to report for
– A End snap id for a snapid range you want to report for
– A Dbid for the database
– The snap interval in seconds (If you have a 30 minute interval input 1800 seconds)

A description of all the column names in the output, broken down by section, is provided in the header section of the script.

There are 6 sections to this script

1) Total Reads + Writes
2) Total Reads
3) Total Writes
4) Read write breakdown for datafiles
5) Data File – Direct Path v/s Buffered Read Write breakdown
6) Read write breakdown for tempfiles

1) Total Reads + Writes

This section displays the number of reads+writes by filetype, and a percentage of reads+writes for each file type, as a percentage of total reads+writes. The last column displays the total reads+writes for all file types. The column DTDP shows the i/o that bypasses flash cache by default and goes directly to spinning disk on Exadata (Temp+Archivelogs+Flashback Logs).