vSphere 4.1 Memory Enhancements - Compression

Finally, with Transparent Memory Compression, 4.1 will compress memory on the fly to increase the amount of memory that appears to be available to a given VM. The new Transparent Memory Compression is of interest in the workload cases where memory -- rather than CPU cycles -- has limitations. ESX/ESXi provides a Memory Compression cache to improve virtual machine performance when using memory over-commitment. Memory Compression is enabled by default when a host's memory becomes overcommitted; ESX/ESXi compresses virtual pages and stores them in memory

Since accessing compressed memory is faster than accessing memory swapped to disk, Memory Compression in ESX/ESXi allows memory over-commits without significantly hindering performance. When a virtual page needs to be swapped, ESX/ESXi first attempts to compress the page. Pages that can be compressed to 2 KB or smaller are stored in the virtual machine's compression cache, increasing the capacity of the host. The maximum size can be set for the Compression Cache and disable Memory Compression using the Advanced Settings dialog box in the vSphere Client.