PLE is a historical average of how long a page remained in cache after having been read in.
The average time a page remains in the cache is two times the time it has been in the cache at any moment.
So (in a stable system were the usage is continuous and same over the the time). A PLE of 400 does it mean that the pages are on average 400 seconds old and will live for an average of another 400 seconds ? Or is the total lifespan of the page in cache 400. So the average time that the page is in cache is 200 seconds and it will live on for another 200 seconds. (Asume a very stable and continuous system).
As for a disfavoured page, it will remain in cache until something else needs cache space. disfavoured just means it'll be the first thing to be tossed out of cache when some other pages need to be cached, instead of been evaluated using the LRU algorithm.
At the moment I am running a checkdb to see if this influences the PLE number. After previous discussions my impression was that dbcc would drop the PLE. And yes it does within 10 minutes the PLE has dropped from 15150 to 189.
The system was not touched for some time so the 15150 was not only the average but also very close to the PLE of each individual page of all pages in the cache. So if the PLE is the average of the time the pages staying in the cache then less then 2 percent of those pages are still there. (Otherwise the average can not become this low.).This leaves me confused.
If the PLE is realy the average then the disfavored system did not work in my situation.
If the disfavored system uses a small part of the cache and reuses that then the PLE is not the actual average of all the pages in cache and also does not predict how long the average page still will stay in cache.Am I missing something ?
Thanks for your time and attention.
Thanks for bearing with me on this.
PLE during the checkdb.
Start at time 0: 15150
After 10 min : 189
Stable for about ? 10 minutes: 186
After those 20 minutes : 207
After 24 minutes : 187