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PLE (Page life expectancy) revisited again.


PLE (Page life expectancy) revisited again.

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ben.brugman
ben.brugman
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GilaMonster (2/22/2013)

ie, if PLE is 400, then on average a page is expected to remain in cache for 400 seconds.


Is that from the moment we measure. Or from the moment the page was last touched ?
(This makes a difference of a factor 2).

What would the PLE be in the example below ?
6 total time that a page is expected to be in the cache.
3 the remaining time that the page is expected to be in the cache.


111111|
22222|
3333|3
444|44
55|555
6|6666
|77777
|888888
|9999999
| AAAAAAA
| BBBBBBB
^
^
time
Above example only 6 pages fit in a cache. The vertical bar is the time line.
Page two gets at that moment replaced by page 8.

Cache now time_left Total_time
1 NO
2 YES 6 0 6
3 YES 5 1 6
4 YES 4 2 6
5 YES 3 3 6
6 YES 2 4 6
7 YES 1 5 6
8 YES 0 6 6
9 NO
A NO
B NO

So the average time (past) the 6 pages are in the cache is 3 seconds
And the average time (future) the 6 pages will remain in the cache is also 3 seconds.
The total time (total = past + future) that a page has remained in the cache is 6 seconds.



Maybe this wil clear up my question.
Sorry that I wasn't clear enough with my question.


Ben
GilaMonster
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ben.brugman (2/22/2013)
GilaMonster (2/22/2013)

ie, if PLE is 400, then on average a page is expected to remain in cache for 400 seconds.


Is that from the moment we measure. Or from the moment the page was last touched ?


If the PLE is 400, then on average a page is expected to remain in cache for a total of 400 seconds. 400 seconds from the time the page was pulled into cache until the time is is removed from cache. On average. PLE = Page Life Expectancy, the time that a page is expected to live in the cache


Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

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ben.brugman
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GilaMonster (2/22/2013)
[quote]ben.brugman (2/22/2013)
[quote]GilaMonster (2/22/2013)


If the PLE is 400, then on average a page is expected to remain in cache for a total of 400 seconds. 400 seconds from the time the page was pulled into cache until the time is is removed from cache. On average. PLE = Page Life Expectancy, the time that a page is expected to live in the cache


Thank you, that does claer it up.
(Sorry that I interpreted the term as REMAINING ple. Probably because I was associating the word expectancy with the future. Maybe a language difference.)

For the checkdb which has finished now, the PLE remained fairly stable. Around 200. Now it is going up again 318 at the moment. It is going up with 1 second for each second. But that's expected. But I still do not know if the cache was completely cleared by the dbcc checkdb.

Thanks,
Ben
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