Memory sizing for linux virtual machines on System z is an important task for optimizing memory usage. z/VM supports the sharing of physical resources to a very high degree. Many virtual machines compete for real memory. Therefore sizing memory for a linux workload is a critical success factor. In a z/VM system the optimal size of a linux virtual machine is typically lower than that of the same workload on a physical server. Linux will satisfy its’ kernel and application memory requirements, and carve up the rest for i/o buffers and file caching. This has been linuxes traditional approach. In the highly shared environment like z/VM ratcheting down memory size is a successful strategy. A good starting point is to decrease memory until linux swapping occurs. Then by careful monitoring and tweaking you can find the sweet spot of virtual machine memory size.