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3.4. Storage Functions

Red Hat Enterprise Virtualization storage components interact to provide storage functions such as:
  • Multipathing allows paths between all LUNs in the Red Hat Enterprise Virtualization environment to be mapped and alternate paths to be determined. This applies to block devices, although the equivalent functionality can be achieved with a sufficiently robust network setup for network attached storage.
  • Provisioning storage allows the logical over-commitment of resources to a virtual machine and the physical assignment of the resource to the virtual machine on-demand.
  • Logical volume extension allows images to be provided with additional storage resources when required.
  • Snapshots provide backups of a virtual machine's system state at a certain point in time.

3.4.1. Multipathing

When the Red Hat Enterprise Virtualization Manager discovers all of the connected LUNs, multipathing is defined over the storage network. All valid paths between the LUNs in the network are mapped and consequently alternate paths are defined in case the primary path fails.
Multipathing protects against a single point of failure and provides increased bandwidth and improved security within the network. All volume groups are created on top of multipath devices, even if multiple paths are not defined for a given device.
Multipathing provides:
  • Redundancy
    Multipathing provides failover protection. If any element of an I/O path (the cable, switch, or controller) fails, an alternate path is found.
  • Improved Performance.
    Multipathing spreads I/O operations over the paths. By default this is done in a round-robin fashion. However, other methods are also supported, including for example Asynchronous Logical Unit Access (ALUA).
Figure 3.4, “Active/Passive Multipath Configuration with One RAID Device” shows an active/passive configuration with two I/O paths from the server to a RAID device. There are 2 HBAs on the server, 2 SAN switches, and 2 RAID controllers.
Active/Passive Multipath Configuration with One RAID Device
Active/Passive Multipath Configuration with One RAID Device
Figure 3.4. Active/Passive Multipath Configuration with One RAID Device

In this configuration, there is one I/O path that goes through hba1, SAN1, and controller 1 and a second I/O path that goes through hba2, SAN2, and controller2. There are many points of possible failure in this configuration:
  • HBA failure
  • FC cable failure
  • SAN switch failure
  • Array controller port failure
A failure at any of these points will cause a switch to an alternate I/O path.