What is "Physical Memory Guaranteed"? When is it effective?
Environment
- Red Hat Enterprise Virtualization 3.x
- Red Hat Virtualization 4.x
Issue
- What is "Physical Memory Guaranteed" in Virtual Machine properties in RHV-M?
- When is it effective to set the "Physical Memory Guaranteed" value different than the default?
Resolution
The "Physical Memory Guaranteed" setting is related to the host selection algorithm. This parameter defines that the host must have at least the "Physical Memory Guaranteed" value of RAM available. Otherwise, the RHV Manager will seek a different host which matches that requirement. For more information how this parameter interacts with the Host Selection Algorithm, please refer to this kbase.
- The "Physical Memory Guaranteed" value can be updated when the VM is on or off, but the change only takes effect when the VM is powered OFF and ON again.
- "Physical Memory Guaranteed" is the lowest value that memory ballooning will reach. This is done by the Memory Overcommit Manager and the ballooning driver. If ballooning is disabled, this value is not used at all.
- Setting a "Physical Memory Guaranteed" amount will cause RHV to report that at least that amount of memory is in use, regardless of actual consumption. So, running
free -mon the hypervisor may not provide a full picture of the amount of memory in use. - In some scenarios, ballooning may collide with KSM. In such cases, the Memory Overcommit Manager will try to adjust the balloon size to minimize collisions. Additionally, in some scenarios ballooning may cause sub-optimal performance for a virtual machine. Administrators are advised to use ballooning optimization with caution.
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