Page Life Expectancy is a Microsoft SQL Server-specific term relating to the average amount of time buffer pages can be expected to remain in memory before being replaced.
From MSDN:
Page Life Expectancy (PLE) is the best indication of how volatile your Buffer Pool is (BP). It's a PerfMon counter, found in the SQL Server:Buffer Manager PerfMon object. Monitor it every 3-5 seconds or so. There is also the Buffer Node:Page Life Expectancy counter which should be considered for NUMA systems, using the same logic per node as one would on a non-NUMA system.
Volatility is measured by taking the average "life" of a page within the Buffer Pool (in seconds). If a page is overwritten or aged out, it starts a whole new life.
So if lots of pages are being overwritten with new data very often, the average PLE will be low, and our BP volatility will be high.
Conversely if most pages in our BP remain there for a long time without being overwritten, the average PLE will be high, and the BP volatility will be low.
So why do we care about BP volatility and PLE? What can knowing the BP volatility do for us? What is a "good" and "bad" PLE figure?
PLE can be a measure of how much physical IO your SQL Server is doing.
