When investigating memory usage, what is the most important measurement for capacity planning purposes?

Resident Set Size (RSS) measures the amount of physical RAM a process occupies, excluding swapped-out pages and shared libraries not currently resident. It is crucial for capacity planning because it directly indicates the true physical memory demand of an application, impacting the server's available RAM.

Total memory refers to the entire RAM installed on the system, which is a static value and not a per-process measurement of usage for capacity planning.

Virtual Memory Size (VSZ) includes all memory a process *can* access, including mapped files and swapped memory, often overestimating actual physical memory consumption.

Swap memory usage indicates memory moved from RAM to disk, which is a symptom of insufficient physical memory rather than the primary measure of a process's actual RAM footprint.