Memory

Memory is used for many things:

• the hypervisor code: this is the Xen executable itself
• the hypervisor heap: this is needed for per-domain structures and per-vCPU structures
• the crash kernel: this is needed to collect information after a host crash
• domain RAM: this is the memory the VM believes it has
• shadow memory: for HVM guests running on hosts without hardware assisted paging (HAP) Xen uses shadow to optimise page table updates. For all guests shadow is used during live migration for tracking the memory transfer.
• video RAM for the virtual graphics card

Some of these are constants (e.g. hypervisor code) while some depend on the VM configuration (e.g. domain RAM). Xapi calls the constants “host overhead” and the variables due to VM configuration as “VM overhead”. These overheads are subtracted from free memory on the host when starting, resuming and migrating VMs.

Related XenAPI Classes, Fields and Messages

Classes

Name	Description

Fields

Name	Description
VM.memory_dynamic_max	Dynamic maximum (bytes)
VM.memory_dynamic_min	Dynamic minimum (bytes)
VM.memory_overhead	Virtualization memory overhead (bytes).
VM.memory_static_max	Statically-set (i.e. absolute) maximum (bytes). The value of this field at VM start time acts as a hard limit of the amount of memory a guest can use. New values only take effect on reboot.
VM.memory_static_min	Statically-set (i.e. absolute) mininum (bytes). The value of this field indicates the least amount of memory this VM can boot with without crashing.
VM.memory_target	Dynamically-set memory target (bytes). The value of this field indicates the current target for memory available to this VM.
host.memory_overhead	Virtualization memory overhead (bytes).
host_metrics.memory_free	Free host memory (bytes)
host_metrics.memory_total	Total host memory (bytes)

Messages

Name	Description
VM.assert_can_boot_here	Returns an error if the VM could not boot on this host for some reason
VM.compute_memory_overhead	Computes the virtualization memory overhead of a VM.
VM.get_cooperative	Return true if the VM is currently 'co-operative' i.e. is expected to reach a balloon target and actually has done
VM.maximise_memory	Returns the maximum amount of guest memory which will fit, together with overheads, in the supplied amount of physical memory. If 'exact' is true then an exact calculation is performed using the VM's current settings. If 'exact' is false then a more conservative approximation is used
VM.set_memory	Set the memory allocation of this VM. Sets all of memory_static_max, memory_dynamic_min, and memory_dynamic_max to the given value, and leaves memory_static_min untouched.
VM.set_memory_dynamic_max	Set the value of the memory_dynamic_max field
VM.set_memory_dynamic_min	Set the value of the memory_dynamic_min field
VM.set_memory_dynamic_range	Set the minimum and maximum amounts of physical memory the VM is allowed to use.
VM.set_memory_limits	Set the memory limits of this VM.
VM.set_memory_static_max	Set the value of the memory_static_max field
VM.set_memory_static_min	Set the value of the memory_static_min field
VM.set_memory_static_range	Set the static (ie boot-time) range of virtual memory that the VM is allowed to use.
VM.set_memory_target_live	Set the memory target for a running VM
VM.wait_memory_target_live	Wait for a running VM to reach its current memory target
host.compute_free_memory	Computes the amount of free memory on the host.
host.compute_memory_overhead	Computes the virtualization memory overhead of a host.