SQL Server Page Life Expectancy (PLE)


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.

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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


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