Active Memory Expansion relies on compression of in-memory data to increase the amount of data that can be placed into memory. This feature expands the effective memory capacity of a POWER7 system. The operating system manages the in-memory data compression, which is transparent to applications and users.

Active Memory Expansion is configurable per logical partition (LPAR) . Active Memory Expansion can be selectively enabled for one or more LPARs on a system. When Active Memory Expansion is enabled for a LPAR, the operating system compresses a portion of the LPAR's memory and leave the remaining portion of memory uncompressed. The memory is effectively broken up into two pools - a compressed pool and an uncompressed pool. The operating system dynamically varies the amount of memory that is compressed, based on the workload and the configuration of the LPAR.