To understand the difference between historical and realtime monitoring you need to understand the difference between In-Memory Monitoring and Repository Persistence. IBM Data Server Manager stores information in two ways.
In-Memory Monitoring is available with Data Server Manager Base Edition and uses the memory of the DSM server to store relatively short term information about your database history. Repository Persistence is available with Data Server Manager Enterprise and allows you to collect and manage a large amount of detailed information about your databases.
You can choose how much data to store in memory by changing your monitoring profile.
In this example, Data Server Manager will collect monitoring information every two minutes and keep the data in memory for one hour. After that time the data is forgotten. If the Data Server Manager service stops running anything in in-memory monitoring will also be forgotten. When you set the volume of data to collect in the Monitoring Profile, there is an estimate of how much memory is required. In this example it is 3.66 MB of data for each database included in this profile.
If you want to keep information longer, you need to setup a DB2 database to store the history. You can specify which database to use through Settings->Product Setup.
The Repository Persistence options in your monitoring profile are different than the In-Memory Monitoring options and allow you to store data for much longer.
In this example we only collect information every 15 minutes but keep it for 31 weeks. We allow Data Server Manager to aggregate data and remove some details after about a week. All these are adjustable but require more storage.
Data Server Manager understands what is "normal"
The Data Server Manager Home page uses In-Memory monitoring to display the current value of key performance indicators (KPIs) for all your DB2 for Linux, UNIX and Windows and dashDB databases. Data Server Manager also calculates what a normal value is for key performance indicators during each four hour window in a week. Normal is within one standard deviation. (About 68% of the values collected fall in this range.) Data Server Manager also saves the historical minimum and maximum values for your KPIs. The most accurate calculation of normal values is available when you have enabled the history repository and have collected a couple of weeks of history. If you only have in-memory data, Data Server Manager will calculate normal based on only the data available in memory. So don't be surprised if you see a lot of out of normal indicators until Data Server Manager has collected enough history. (The Normal Range Bar shown below is new for Data Server Manager 2.1.4.)
Both real-time and repository monitoring data are used in the Data Server Manager Monitor page. Select Monitor->Database.
Realtime monitoring displays a picture of the current database status. The overview page shows a view of the last hour of time spent and key performance indicators like CPU usage and transaction rates. Everything is either an immediate view of the current state of the database or a view of no more than the last hour. The normal range for the current 4 hour window is shown as a light blue background.
To see data past one hour, select History from the Monitor page. History can provide a much longer look back into the past. It uses both the in-memory monitoring data as well as data stored in the repository database. In the example below we are looking at the last three hours of history. The first hour contains samples every few minutes and you can see the additional spikes from the high variability in this database. After the first hour, data is only collected four times an hour. So while fewer spike appear in the graph that is likely only because of the less frequent monitoring.
Normal in History
In history mode, the light blue background shows the normal range (one standard deviation from the average) of the period being displayed. In this example about three hours. The longer the period being displayed, the more data is used to calculate the normal range.
DSM Base vs Enterprise
If you have Data Server Manager you can see historical data, but only what is available through In-Memory monitoring. Use of the Repository database is only available with Data Server Manager Enterprise included with DB2 Advanced Editions and for use with dashDB.