This week I was involved in a conversation with Paul Randal relating to PLE per node vs PLE server wide.
There is an all-up PLE counter as well as individual, per NUMA node PLE counters. SQL Server Books Online describes the values as:
SQL Server Buffer Manager \ Page life expectancy – Indicates the number of seconds a page will stay in buffer pool without references.
SQL Server Buffer Node \ Page life expectancy – Indicates the minimum number of seconds a page will stay in buffer pool on this node without references.
The descriptions leave a bit to the imagination. It is pretty common place to ask someone about the all-up value and the assumption is a simple average of the individual node values. For example, using the following 4 Node values, the AVG = 1750 divided by 1000 = 175.
This is not the calculation used for all-up number. The Buffer Manager value is an average of the rates or the (Harmonic Mean.) Using the harmonic mean, run rates average, for this example the PLE = 155.
Paul outlines additional calculations and highlights the need to watch per node values for better management of your PLE targets in his post: http://www.sqlskills.com/blogs/paul/page-life-expectancy-isnt-what-you-think
Bob Dorr - Principal SQL Server Escalation Engineer