Alexander Suprun (1/22/2013)
ben.brugman (1/22/2013)
But how is it possible that SQL-server Expects that the memory can be refressed (8Gb of it) in 14 seconds ?I do not think normal disk IO (SAN) can handle that amount of IO.
How is the PLE determined?
Why don't you stop inventing your own definition of PLE and read the documentation?
http://msdn.microsoft.com/en-us/library/ms189628.aspx
Page life expectancy - Indicates the number of seconds a page will stay in the buffer pool without references.
The definition status that PLE is a prediction.
Predictions often are based on something from the past.
So I would like to know how PLE is determined.
And based on the above please try to explain what PLE has to do with disk IO and SAN?
I have seen very low PLE's starting this thread 7 was the lowest. (Now I have seen even lower). A Page Life Expectancy of 7 seconds for 8 Gb memory, suggest that the next 14 seconds all pages in Memory will be replaced by reading in new pages. (That is 8 Gb in 14 seconds this is over 500 Mb per second. In real life a disk (even a SAN) can not deliver this amount of data.
Other scenario's are possible, but for the other scenario's the number of bytes read per second is even higher. To get to an PLE of 7 an enourmous amount of data has to be replaces in a short time.
(I asume the PLE is some sort of average).
So to understand what PLE means, I would like to know how this prediction is calculated. (My expectation about the lowest a PL (average Page Life) can become is higher than 7)
Thanks for your reaction,
Ben Brugman
Additional, (It is a bit more complex)
The PLE of 7 actually does not expect a page to be replaced within an average of 7 seconds it expects a page to be replaced or touched (referenced) within those 7 (or is it 14) seconds. The text above ignores the fact that a large number of pages can be referenced within the timeperiod.