In a nutshell, Capacity Management is all about modeling an IT system (i.e. applications and infrastructure) so that we can answer business questions such as:
When facing typical three-tier enterprise applications, creating accurate models is increasingly difficult as we move from the front-end layer to the back-end.
Front-ends (i.e. web servers) and Mid-ends (i.e. application servers) can be modeled with relative ease: their resource consumption is typically driven by user activity in business-related terms (i.e. business transactions per hour, user visits per hour etc). The chart below shows a typical linear regression model for such a system: the application servers CPU shows a good linear relationship with the volume of purchase orders passing through the application. The high correlation index (R-squared value next to 1) is a statistical indication that orders volume explains remarkably well the pattern of CPU utilization.
When you deal with the back-end (i.e. database) component of an application, things are not as straightforward. Databases are typically difficult to understand and model as they are inherently shared and their resources pulled in several directions:
The next chart shows a typical relations among the total CPU consumption of a shared database server vs. the business volume of one application. This model is evidently quite poor: a quick visual test shows that the majority of the samples do not lie on or close to the regression line. From a statistical standpoint, the R-squared value of 0.52 is well below normally acceptable levels (commonly used thresholds are 0,8 or 0,9).
The end result is: simple resource consumption vs. business KPIs models do not work for databases. This is unfortunate as databases are usually the most critical application components and the ones customers are most interested in properly and efficiently managing from a capacity standpoint.
I will now show you a methodology that we have developed and used to solve challenging database capacity management projects for our customers. The solution lies in a better characterization of the workload the database is supporting. In other words, we need to identify the different sources of work placing a demand on the database (e.g. workload classes) and measure their impact in terms of resources consumption (e.g. their service demands).
If the database is accessed by different applications (or functions), we need to measure the resource consumption of each of them (e.g. CPU used by application A and B, IO requests issued by online and batch transactions and so on). Once we have that specific data, we are in a much better position to create capacity models correlating the specific workload source (i.e. online activity) and the corresponding workload intensity (i.e. purchased orders per hour).
That’s the theory, but how can we do it in practice? Keep reading and you will learn how we can take advantage of Oracle database metrics to accomplish this goal!
Starting from release 10, Oracle has added a considerable amount of diagnostic information into the DBMS engine. It turns out that this same information is very helpful also from the capacity management perspective.
It is possible to extract CPU and I/O resource consumption by instance, schema (database user), machine or even application module/action/client id (Module/action/client id information is available provided that the application developer has properly instrumented it. See for example: http://docs.oracle.com/cd/B19306_01/appdev.102/b14258/d_appinf.htm ).
Oracle tracks resource consumption on a variety of ways. The most important ones, from the capacity management perspectives are: per session and per SQL statement basis. The former metrics can be retrieved from V$ACTIVE_SESSION_HISTORY, the latter from V$SQLSTATS. The corresponding DBA_HIST_<VIEW> can be queried for more historical samples.
The idea is to:
II’ll show you how Oracle work of an instance can be broken down by the application accessing it using session metrics.
In my experience per-session metrics proved to be more accurate than per-SQL statement metrics. Although the latter metrics measure actual SQL resource consumption (no sampling), the drawback is that they might not track all the relevant SQL statements that the database executed. Indeed, SQL statements might go out of library cache and therefore not being counted in AWR views and reports! See for example: http://jonathanlewis.wordpress.com/2013/03/29/missing-sql/
Step 1 is a function of how the application connects to the database. In this example, a single instance database was being accessed by different applications each using a specific user.
Step 2 is more interesting. How can we aggregate the application sessions to get the total number of CPU consumed? The Oracle Active Session History view samples session states every second. A possible approach is to:
It is important to note that this is a statistical measure of CPU consumption and therefore might not be 100% accurate. Always double check totals with resource consumption from other data sources known to be correct (i.e. monitoring agents or the operating system).
An example query that does this calculations can be found here:
SELECT TO_CHAR(sample_time, 'yyyy/mm/dd HH24') as sample_time, username, round(sum(DECODE(session_state,'ON CPU', DECODE(session_type,'BACKGROUND',0,1),0))/60/60*10,2) AS cpu_secs FROM DBA_HIST_ACTIVE_SESS_HISTORY a, dba_users b WHERE sample_time > sysdate - 7 and a.user_id=b.user_id GROUP BY TO_CHAR(sample_time, 'yyyy/mm/dd HH24'), username order by 1,2
By using the above outlined methodology, I have isolated the database CPU consumption caused by the work placed by a specific application I was interested in (for example, Order Management). The next chart shows the relation among the physical CPU consumption of the database (caused by our selected application) vs the business volume of the selected application. The model now works great!
I hope you now agree with me that workload characterization is an essential step in capacity management. If properly conducted, it can provide you with remarkably good models when they are most needed!
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