Performance considerations for DDM

 

These topics provide information to help you improve performance when using DDM and also provide some information about when to use something other than DDM to accomplish some functions.

In general, the processing of remote files by using DDM is transparent to an application program or utility function, such as that provided by the Copy File (CPYF) command. However, additional time is required when accessing remote files by using a communications line. The performance difference between local file and remote file processing is proportional to the number of accesses to remote files, the data record length, and the line speed during a unit of performance measurement.

An additional difference between local and remote file processing is that the input or output operation to a local file might not result in an immediate physical disk operation because the server transfers blocks of data from the disk and writes blocks of data to the disk. There are times, then, that the user program accesses data within main storage and the physical I/O occurs at a different time. Therefore, to minimize the difference between local file and remote file performance, it is essential that knowledge of an application design and the amount and type of accesses to files be considered when determining which files are to be accessed remotely using DDM.

The additional time for each remote access is comprised of:

The communications line time accounts for most of the additional time, though the actual time is dependent on line speed and the amount of line activity during the DDM function.

As is true in non-DDM cases, local and remote server job priorities have the most significant effect on performance. On an iSeries server, the PRIORITY and TIME SLICE values of the class being used control job priority. The SDDM runs under the source job, and the TDDM runs under the class assigned to the APPC routing entry of the target server's subsystem. In applications that access multiple files, the best results are achieved when the most heavily accessed files are on the same server as the program that is running and the less heavily accessed files are on a remote server. Key considerations regarding the placement of files and application programs follow:

Consider two sample application processing techniques, batch file processing and interactive file processing.

 

Parent topic:

Operating considerations for DDM

 

Related concepts


Security

Communications Management PDF
APPC, APPN, and HPR

LAN, Frame-Relay, and ATM Support PDF

 

Related reference


Display DDM remote file records
User exit program considerations for DDM