i was just adjusting the amount of shares on all of our ressource pools to get the right balance between of %-shares for the number of VMs per pool.
By doing this I noticed that all pools had a "small" memory reservation. First I thougt "I never set any memory reserverations!?". I tried to set the reservation to 0 and got the message that the minimum value must be xxxx MB.
After thinking about it and looking up in Ducans & Franks "Clustering technical deepdive" it was clear that the minimum reservations is the sum of all the mermory overhead of the VMs bellow my RP.
The reason why the value was 0 in the past (and still showing in the "ResourceAllocation"-Tab until opening the editing window), is that if you create a new resource pool there is of course no VM in it and so there is no overhead which needed to be reserved. If you going to edit a resource-pool after you moved VMs into it, the minimum reservation of memory is set equal to the current overhead of these VMs.
Ok...now coming closer to my question ....
-I create a RP named "Test-Pool". Mermory Reservation = 0 MB
-Move 2 VMs with each 32GB! RAM and 2 VMs with each 2GB into this Pool
-Click Edit Setting on the RP and the min value of memory reservation is around 2GB for this 4VMs
-I move the 2 VMs with the 32GB into another RP.
-Checking the RP setting of "Test-Pool" and the mem reservations are still at 2GB (ok now i have the possibilty to set it back to ne min needed mem for the 2 VMs with 2GB (~200MB).
Suming up I am ending with a 2GB reservation on my RP which I never wanted to have, because "the automatic adjustment of the mem reservation" is just working in one direction.
Of course I know that unused reservations can still be used by other VMs, but if contention happens this kind of reservations could led to results that are not wanted (especialy in big environments with VMs with a lot of RAM).
So does this mean to me that I always need to check the mem reservation on all my pools after moving some VMs accros RPs or did I get something wrong?