A reservation
is typically set against a resource pool and filters down to give a VM rights
against memory. Essentially if a
reservation has been set and applies to this VM, then the VM is guaranteed that
amount of memory will be made available in RAM on the EX host. You can never reserve more memory than
exists. So reservations can ensure good
performance for the VMs you care about.
You put the VMs in a resource pool, and allocate a reservation that’s
appropriate. That might be your 1:1
ratio with allocated and reservation.
Then let other less important VMs worry about oversubscription.
When an ESX
host is very short of memory it may have to resort to using .vswp swap files
for the VM memory. At this point
performance will be affected as data that the OS believes is in memory is, in
reality, now on disk.
A VM as
default can have up to 65% of its memory used by the balloon driver. It may also have a memory reservation. The reservation cannot be swapped or taken up
by the balloon driver. Any memory
outside the 65% used by the balloon driver,
and the reservation, can be placed into a .vswp file.
In reality
you never want this to happen.
On Wednesday I'll be discussing Memory.
Phil Bell
Consultant
