Split I/Os and Window Turns affect System Performance
What is a Split I/O? What is a Window Turn?
How can I confirm that decreasing system performance and longer system response time is due to fragmentation?
The Monitor facility allows you to observe Window Turns and Split Transfers. Split I/O's or Split Transfers are a direct measure of fragmentation. A split I/O transfer occurs when a single logical I/O request spans multiple physical I/Os. The monitor in OpenVMS V4.x does not tell you about Split I/Os, but the 5.0 monitor does. OpenVMS V4.x keeps track of the number of Split I/Os it has performed since the system was last rebooted. The address where it keeps this is named "PMS$GL_SPLIT". The address is 80000B1C (hexadecimal). You can look at the split I/O count like this:
$ EXAMINE/DECIMAL 5X80000B1C
Split I/Os waste computer resources. Every I/O uses CPU time. Therefore, when I/Os are split your CPU is busier and other processes have to wait. Every extra I/O also costs elapsed time, which means that your own jobs take longer to run. If you are using OpenVMS V5.0 or later, you will be able to see Split I/Os occuring on your system by typing:
$ MONITOR IO
and looking at the split transfer rate.
A window turn occurs when there are not enough file pointers in the window to tell the system where to look for the other parts of the fragmented file. For example, if a window can only hold seven file pointers, and there are ten fragments, it will take two window turns to retrieve the file, one window for the first seven file pointers and the second window for the remaining three. Therefore, if you have window turns, you are suffering from fragmentation.
OpenVMS 4.x keeps track of Window Turns in the same way it keeps track of Split I/Os. The address where this number is kept is called "PMS$GL_TURN". The address is 80000B18 (Hexadecimal). Look at this value in this way:
$ EXAMINE/DEC %X80000B18
In OpenVMS V5.x and later, you will be able to see "Window Turns" occuring on your system by typing:
$ MONITOR FCP
and looking at the Window Turn Rate.
In order to use the count, multiply the number of Window Turns by the time it takes for an I/O. The amount of time it takes for an I/O on your VAX depends on the type of disk and controller you have. Remember, each Window Turn means one extra I/O, so multiply Window Turns by I/O time to get the total time wasted.
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