Bug 1993128

Summary: [oVirt][Doc] Document guaranteed memory minimal requirements and known issue for RHV 4.4.7
Product: OpenShift Container Platform Reporter: Gal Zaidman <gzaidman>
Component: InstallerAssignee: Eli Marcus <emarcus>
Installer sub component: OpenShift on RHV QA Contact: Michael Burman <mburman>
Status: CLOSED CURRENTRELEASE Docs Contact:
Severity: high    
Priority: high CC: apinnick, ctomasko, emarcus, mburman
Version: 4.9Keywords: Documentation
Target Milestone: ---   
Target Release: 4.10.0   
Hardware: Unspecified   
OS: Unspecified   
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Last Closed: 2022-03-24 12:14:40 UTC Type: Bug
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Bug Depends On: 1993017    
Bug Blocks:    

Description Gal Zaidman 2021-08-12 12:46:30 UTC 
Version: All openshift versions
Platform: oVirt
Please specify: IPI

What happened?

On RHV 4.4.7 the default "guaranteed memory" for a newly created VM was changed.
- On RHV < 4.4.7: The guaranteed memory will be taken from the template.
- On RHV = 4.4.7: The guaranteed memory will be equal to memory without taking memory Overcommitment into account.
- On RHV > 4.4.7: The guaranteed memory will be equal to memory *BUT* with taking memory Overcommitment into account, meaning if you created a VM with 10GB mem and have 150% memory overcommitment set on the cluster level then you will get 10GB*0.66 = 6.6GB of guaranteed memory (See BZ#1993017[1]).

For Openshift that change the behavior of the installation, that means that:

- On RHV 4.4.7 all the machines will be created with a guaranteed memory equal to memory, which means that if the user relied on having memory overcommitment in RHV to have enough resources Openshift then the installation will fail due to not enough memory.

- On RHV 4.4.8 and above (when BZ#1993017[1] is fixed) the VMS will still be created with a guaranteed memory set but at least with a value that makes sense for memory overcommitment - so there is no problem with that.

What do we need to do?

1. Document that on RHV 4.4.7 there is a known issue that affects users that rely on memory overcommitment where each VM is created with a guaranteed memory == memory - installation or new machines will not be created if there is not enough *physical memory* in the cluster.

2. Document that on *Openshift 4.9* and above you can set the guaranteed memory on MachineSet as a day2 operation - if you don't set it the behivour is dependent on your RHV version.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1993017