6. Memory Virtualization
PowerVM also offers various configuration options in the area of memory virtualization. The most frequently used variant is the use of dedicated memory, whereby physical memory areas are permanently assigned to an LPAR. Only one LPAR can then access the assigned memory areas. PowerVM also supports the possibility of sharing memory. A number of LPARs share the physical memory in a memory pool. Of course, only one LPAR can use a physical memory area from the pool at a time, but unused memory is automatically returned to the memory pool by an LPAR and can be used by another LPAR with currently higher main memory requirements. By sharing the physical memory, it is used more efficiently, which usually means that all LPARs together require less overall main memory. The sharing of memory is known as Active Memory Sharing (AMS).
Another way to save physical main memory is to use Active Memory Expansion (AME). A system is assigned a certain amount of physical main memory, together with a memory expansion factor. From the perspective of the LPAR, more main memory is then available than has actually been physically assigned. This is achieved through the support of the operating system, which gains the additional memory by compressing memory pages that are currently not in use. For example, an LPAR can access memory of 2.0 * 32 GB = 64 GB with an assigned physical memory of just 32 GB and an AME factor of 2.0. The additional memory that becomes available through compression must ultimately be paid for by using more processor resources for compression and decompression. But there are a number of areas of application where this works very well.