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3.4.2. Provisioning Storage

The Red Hat Enterprise Virtualization Manager provides provisioning policies to optimize storage usage within the virtualization environment. Thin provisioning policies allow administrators to over-commit resources and provision storage based on the actual storage usage of their virtualization environment.

3.4.2.1. Over-commitment

Thin provisioning is a storage policy that allows storage to be allocated to virtual machines on an on-demand basis, enabling the over-commitment of available resources. While the Red Hat Enterprise Virtualization Manager provides its own thin provisioning function it is recommended that thin provisioning functionalities provided by the storage back end for a given environment be used instead.
Over-commitment is a storage function which allows the Red Hat Enterprise Virtualization Manager to logically allocate more storage than is physically available. Generally, virtual machines use much less storage than what has been allocated to them. Over-commitment allows a virtual machine to operate completely unaware of the resources that are actually available to it at a given time.
The Red Hat Enterprise Virtualization Manager uses VDSM to define a threshold within the logically assigned storage which is used to determine that the resource usage remains within the bounds of the storage available. QEMU identifies the highest offset written to in a logical volume, which indicates the point of greatest storage use. VDSM monitors the highest offset marked by QEMU to ensure that the usage does not cross the defined threshold. So long as VDSM continues to indicate that the highest offset remains below the threshold, the Red Hat Enterprise Virtualization Manager knows that the logical volume in question has sufficient storage to continue operations.
Because a disk image can be logically defined with more storage than is physically available to a logical volume, usage can rise to exceed the threshold limit. QEMU indicates when the storage usage exceeds the threshold, which means the logical volume assigned the storage will soon run out of physical storage. VDSM then requests that Red Hat Enterprise Virtualization Manager request the SPM to extend the logical volume. This can continue as long as the data storage domain for the data center has available space. When the data storage domain runs out of available free space, the administrator must manually add storage capacity to expand the volume group. For details about logical volume extension, refer to Section 3.4.3, “Logical Volume Extension”.