Execution of a storage space request may lead to saturation states in the following areas: main memory, system address space and paging memory.
Memory bottlenecks can also occur in the following situations which are described elsewhere in this manual:
If the main memory load or migration rate is too high (see "Allocation of resources")
If storage space is requested on public volumes (see "Monitoring storage space saturation")
The messages output in accordance with the individual saturation states and causes are listed in the following sections. The same also applies to the measures that can be performed either by the operator or by system administration. This section focuses on long-term measures for the prevention of saturation states.
Main memory
The main memory size should be determined in accordance with the recommendations in the Performance Handbook. If the main memory is too small, this usually has a negative effect on performance since the paging rate and displacement rate are too high.
In the case of VM2000 guest systems, the recommendations in the Performance Handbook concerning the value of MINIMAL-MEMORY-SIZE must be observed.
The MODIFY-SYSTEM-BIAS command makes it possible to allow user programs to make a larger number of user programs resident. It is advisable to restrict this authorization to user IDs under which such programs are to run (ADD-USER and MODIFY-USER-ATTRIBUTES commands).
System address space
The system address space requirement is highly dependent on the system configuration and peripheral devices as well as on the employed subsystems and system load.
You are strongly advised to be generous in assessing the system address space (the size can only be set on SUs /390, see "System address space").
An insufficient system address space can result in undesired interruptions to operation (e.g. because of subsystem version changes, increases in load, extension of the system configuration or because of the after-effects of a saturation state).
You are advised to use the available load limitation possibilities. For example, you can limit the number of users in the job classes (MODIFY-JOB-CLASS command). A user-specific limitation of the program size is also possible (by means of the ADD-USER and MODIFY-USER-ATTRIBUTES commands).
Paging memory
The size of the paging memory should be chosen on the basis of the recommendations in the Performance Handbook.
A sufficiently large paging memory must be set up during system initialization (parameter service).
If the paging memory requirement grows significantly during a session then the extension should be performed as early as possible (by means of the EXTEND-PAGING-AREA command).
The possibilities for load limitation should be used (see “System address space” above)
To prevent performance degradation, algorithms which are designed to counteract the saturation state come into effect if saturation conditions occur.
In these circumstances the Control System displays the messages contained in the tables below on the console.