Memory Reservation in a VM and its impact

[A25Simon]

I want to provide a VM sure shot memory of arounbd 10 GB upfront.... and this is testing phase... can i revert the reserved memory once the testing is completed ? Can i disable reservation ?

(I am running a SAP application that needs high volume of memory upfront... )

[a_p_]

Yes, you can set/reset the reservation any time you want with the VM powered off. If the VM runs on a host in an HA cluster you need to be careful with HA admission control. If it is set to "Host failures cluster tolerates" the reservation is taken into account when calculating the slot sizes!

Anyway, a reservation may only be needed if you are low on memory. If the host has enough physical memory to satisfy the VM's configured RAM, there's no need for the reservation.

André

[A25Simon]

That’s the problem, HA does not allow me configure the VM with greater RAM. I need 24 GB but HA is allowing me only 16. So I thought a memory reservation should be the workaround. Alternatively I can try disabling HA admission control so that it allows the VM to power on the high memory VM (24 GB)

What do you recommend ?

[a_p_]

I don't know you environment. However, if HA prevents you from powering on the VM with a high reservation, you may either disable Admission Control while testing or change the settings to a percentage (e.g. 100 / <number-of-hosts>).

André

[rickardnobel]

Aroop V Simon wrote:

That’s the problem, HA does not allow me configure the VM with greater RAM. I need 24 GB but HA is allowing me only 16. So I thought a memory reservation should be the workaround.

Depending on your Admission Control Policy, doing reservations will only make things worse. As mentioned before, the largest reservation (which might be this particular VM) impacts the so called "slot size", which HA uses like a template for how large a VM is. The most possible outcome is that you will get less VMs that are allowed to be powered on.

[shishir08]

Yes you can set the reservations for the VMs and once done you can remove the reservations as well but if the VM runs in HA cluster then you have to  take into consideration below advised points.

Failover Capacity is determined using a slot size value that is calculated on the cluster. Slots are calculated by a combination of the total CPU and Memory that are in the physical hosts.

some of the workaround/way to fix it.

• Set the “Allow Virtual Machines to be powered on even if they violate availability constraints” in the configuration of the cluster. In this case it ignores the aHA calculation and will try to power on as many VM’s as possible in case of HA failover. If this is the option chosen you can also set restart priority in the ‘Virtual Machine Options’ section of the cluster configuration. This way any high priority VM’s are powered on first, and then the lower priority up to the point where we cannot power any further VM’s on
• If you have one VM which is configured with a very high amount of memory, you can either lower its configured memory, or take it out of the cluster and run it on any other standalone ESX host. This will increase the number of slots available with the current hardware
• Increase the amount of RAM on servers so that there are more slots available with the current RAM reservations.
• Remove any CPU reservations on any VM(s) that are greater than the max speed of the processors in the hosts.