Similar to the case of shared processors, the physical memory can also be shared between several LPARs. Different amounts of physical memory are assigned to the LPARs at different times, depending on their requirements. The amount of physical memory currently assigned to an LPAR can vary over time.
As a rule of thumb, the main memory size of an LPAR is chosen by the administrator to be sufficient for the peak utilization, usually with a smaller or larger safety buffer. The so-called working set (memory pages that are actually used) is usually smaller. Otherwise, increased paging will result. With PowerHA systems, a sufficient amount of memory must be available for applications currently running on other nodes. Otherwise the failover in the case of an error could fail because there is not enough memory available to start the additional applications from the failing node. However, this means that, especially in cluster systems, the difference between the memory actually required and the configured memory can easily be only half the main memory size. As long as there is no failover, the memory configured for this additional applications is not used (apart from file system caching).
Figure 6.2 shows the actual memory usage of some LPARs over the course of a day. The red line shows the sum of the configured main memory sizes of the LPARs (248 GB). In the course of the day, however, the total memory usage of all LPARs together, remains most of the time significantly below this value. The values shown come from real production systems. Although one does not see the mountains of use of an LPAR, which are often shown in this context, which meet a valley of use of another LPAR, between approx. 60 and 100 GB of allocated memory are still not used by the LPARs. This memory could be used, for example, to run further LPARs on the managed system, or to assign more memory to individual LPARs.
You must be logged in to post a comment.