What Is Your Definition of Database Workload?
January 8, 2009
Providing a definition for database workload can be a daunting task. Unfortunately, it depends on what youre trying to accomplish.
Early in my database administration days, I purchased a book entitled The Art of Computer Systems Performance Analysis. This book, back then and probably now too, has been very influential in my understanding of how to tune not only databases but complete systems. Part of the complete picture of performance evaluation is picking a workload that is appropriate so that you can effectively understand if your system is performing well, if your tuning efforts are working, and if you can scale a system. With this article, Id like to dive into what a workload is as there seems to be some confusion in the database community.
To me, it is always interesting to check a dictionary definition first. These are not always right within the context of database systems but do tend to serve a purpose (which we will see). So, from dictionary.reference.com we get the following:
This, to me, is a very interesting definition and we should not be confused or sidetracked by this. When evaluating this definition we must keep in mind that while a machine is mentioned, this in no way relates to a computer or even a database. This machine is static, has no options or chance of improved performance. For instance, if we look at a copier machine, a copier can copy just so many pages per minute regardless of how high we stack the paper. For a closed and static copier machine, the workload/load/work produced is always the same.
When we look at computer systems, workload takes on a completely new meaning. Workload is, in its purest sense, the total requests made by users and applications of a system. For an Oracle database, the workload would be all of the SQL statements that have been submitted to the instance to work on regardless of being executed, returning results back to their respected users, or even actually being seen within the Oracle database itself. This concept is very difficult for many to understand--mostly because it is hard to tune anything we cannot see. After all, we as database administrators can only tune what we see.
Therefore, as database administrators, when all we are working with is the database, we must begin to take off our purist hats and take a more liberal definition on what workload means to us. In this case, we need to define workload as what the database sees regardless of what is happening outside the database. This means we must sample, measure, track, etc. those statistics within Oracle that measure, quantify, or characterize what the workload is. It is critical that we choose the statistics within Oracle that clearly measure workload opposed to those statistics that depend on the performance of our database system. For instance, statistics like CPU usage, elapsed time, and various wait event times would not be good examples of measuring a workload as they depend too greatly on the database system that SQL statements are being executed on. More appropriate workload metrics would be groupings for things like number of SQL executions, active sessions, users, or applications.
When we define a workload, regardless of that workload metric, we must ensure that the workload we track can be related to the resources being consumed within our database systems. When we do this, we can then tell what performance impact is experienced, or will be experienced, based on increases or decreases of workloads.
Ok, now I know what you are thinking here. Youre thinking that since Ive just tied workload to resources being used, why cant I just measure the resources being used and forget about taking the purist point of view for workload? Well, you are right in one way and wrong in another.
When you look at a database, it really is a closed system in a sense. We have a set number of parameters that are static that interact with mostly static external resources such as CPU, memory, and disk. As a static system, as we mentioned previously in the copier example, it doesnt matter how man applications we stack against our database it will only be able to respond and provide result sets back to an application or user. Thus, our consumption of resources within the database can nearly be thought of as workload metrics; and if youre only concerned about tuning your database this is 100% ok in my eyes.
However, for instances where you might want to do some scalability testing or port to a new platform it is really hard to determine if scalability or performance is better, worse, or indifferent. This is where tying to conform to a more purist workload definition is needed. Look at Oracles Database Replay feature. While I have not used this feature, it is exactly what this use case suggests. You need to capture a workload from a system and then port that workload to another system for testing, benchmarking, load analysis, etc. This is the only way to say if one system will perform better than another will.
That being said, if you look at Oracles Database Replay feature, you will notice that even it does not hold to a complete purist point of workload capture. It too is restricted to capture only those requests seen by the database. Not only that but Database Replay has some restrictions that might skew or negate any attempts to benchmark workloads on other systems.
So, what are we left with? It seems to me that regardless of how you define workload, the only real imperative is that you are able to, at the end of the day, be able to tie resources consumed within the database to higher level SQL statements, users, applications, server farms, etc. Those high-level ties provide extreme power in tuning a database properly.