How much RAM can be reservated for a single VM

[crispyduck00]

Hi,

I have a 6.5u1 standalone host with 10GB of RAM, and one VM on it. As there is a passthrough device assaigned I would like to reservate as much as possible RAM to this VM, but as soon I increase the reservation above 4048MB  following error appeares when trying to start the VM:

The available memory resources in the parent resource poolare insufficient for the operation

I tested this also with 8GB RAM, there I was only able to configure 3GB reservation.

So why is it not possible to assign more reservated ram to a VM?

Thanks,

Crispyduck

[daphnissov]

Because ESXi needs memory as well. Why are you using reservations in the first place? More than likely, you don't need them in your setup.

[Yuva_1990]

Hi

Please find the below community article which might be helpfull

memory reservation

Regards

Yuvaraj

[crispyduck00]

I have to reservate all VM Memory as this VM has attached a passthrough device.

I know that ESXi also needs some memory, but not 6GB of RAM when there is a single VM with one vCPU with 4GB RAM?

[crispyduck00]

Hi,

@Yuvaraj: Thanks, I already read this article, and I am aware of memory overhead, but I thought it should at least be possible to reservate half of the host memory to a single VM with just one vCPU.

As I said, there is and will be only one VM on the host with a passthrough device attached, therefore it is necessary tp reservate all VM memory.

With 8GB RAM it is not possible to start the VM with more then 3GB reserved. That are just 3GB vor the VM and 5GB for ESXi and overhead.

When I added 2GB RAM to see how it behaves then it was also not possible to go beyond 4GB. So just 4GB for the Vm and 6GB for the ESXi and overhead.

Br,

ceispyduck

[daphnissov]

You do not need to reserve memory if this is the only VM on the host, even if you are passing through a device.

[crispyduck00]

With a FPT device it is necessary to reserve all memory that is assigned to the VM, or the VM will not start.

See KB:

VMware Knowledge Base

[ashwin_prakash]

Thank you for the update, We would like to inform you that the KB that you are referring to is for ESXI 4.X.

There is not such known issues with ESXI 6.5, For Powering ON Virtual Machine with a Specific Reservation.

If you would like to reserve the complete available memory on the Host.

First you would have to increase the Memory Allocated on the Resource Pool.

Once the Resource pool has been allocated with the required Memory, then you would be able to Reserve or allocate the memory on the Virtual Machine.

[crispyduck00]

Hi I know this KB is for 4.x but it produces the same error on 6.5u1 when not all memory is reserved for the VM:

memory reservation (sched.mem.min) should be equal to memsize(xxxx)

I found also somwhere else that for 6.5 the reserve all vm memory checkbox has to be marked for VMs with a FPT device. And I think this must be correct, as I always get the (sched.mem.min) error if not all memory is resereved.

I was already thinking of resource pools, even if there are no more resource pools in the integrated host client. 😞

How to change a resource pool on a standalone host?

And is it even possible to change the root resource pool?

Thanks,

crispyduck

[crispyduck00]

So does someone know how much of the physical host ram can be reserved for a VM on a standalone ESXi 6.5u1?

Is it really just about 40%?

With 8GB RAM it was only possible to reserve 3Gb, with 10GB RAM it was only possible to reserve 4GB, how much would it be with 32GB or 64GB?

Br

crispyduck

[daphnissov]

The bottom line is if you must reserve that much memory to a VM, you will need a higher density of memory available to your ESXi host.