Central and expanded storage

The system data mover (SDM) uses central storage for the host sidefiles that it maintains for concurrent copy sessions. A sidefile is a storage area that is used to maintain copies of tracks within a concurrent copy domain. A concurrent copy operation maintains a sidefile in storage subsystem cache and another in processor storage. The MVS real storage manager may page sections of the sidefile into expanded storage in response to varying system loads.

The size of the host sidefile is a secondary consideration when using concurrent copy. The SDM uses the sidefile only to store temporary copies of tracks as part of the intercepting write process. The size and use of the sidefile relate directly to the number of intercepted writes. The size of the host sidefile is unlikely to be a significant factor unless you use a concurrent copy operation during a period of very high update activity or in a very storage-constrained environment.

The SDM creates one host sidefile for each concurrent copy session. The host sidefiles for different concurrent copy sessions share a single data space, which can grow to a maximum size of 2 GB. The actual size of the data space is dynamic, however, and changes in response to the SDM requirements. The size depends on the following considerations:

The number of active concurrent copy sessions
The level of update activity
The rate at which DFSMSdss reads data and writes it to the output media

In the unlikely event that the sidefiles will fill the data space, the SDM creates additional data spaces as necessary.

Another factor that affects the host sidefile size is the distribution of application writes across a device. I/O operations on a device typically concentrate on a set of tracks rather than uniformly distributing updates over the device. An application that updates a track causes an intercepted write. Subsequent updates to the same track do not result in an intercepted write. Similarly, as soon as DFSMSdss has processed a particular track, updates to that track do not result in an intercepted write. Concurrent copy only uses the host sidefile when there are intercepted writes. Therefore, the grouping of I/Os by applications significantly reduces the potential maximum size of the host sidefile.

Running many concurrent copy jobs simultaneously can cause auxiliary (AUX) storage shortages. You can control the total amount of AUX storage that is used by concurrent copy jobs at any given time. The MVS System Resources Manager (SRM) uses two different percentages (a lower value and an upper value) to determine when AUX storage shortages have been reached. SDM provides the ability to modify the concurrent copy AUX values that are used in evaluating the AUX storage that is used by concurrent copy. During concurrent copy processing, SDM subtracts these values from the two MVS percentages to determine if the total current system AUX storage percentage being used by the system is above these newly computed SDM percentages.

You can use the MODIFY MVS system command to change either the new or the existing concurrent copy job AUX delta values, as shown in Example: specifying concurrent copy job AUX deltas.