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# Glossary

ballooning
  A term describing runtime, dynamic changes to an instance’s memory, without having to reboot the instance. Depending on the hypervisor and configuration, the changes need to be initiated manually, or they can be automatically initiated by the hypervisor based on the node and instances memory usage.
BE parameter
  BE stands for backend. BE parameters are hypervisor-independent instance parameters such as the amount of RAM/virtual CPUs it has been allocated.
DRBD
  A block device driver that can be used to build RAID1 across the network or even shared storage, while using only locally-attached storage.
HV parameter
  HV stands for hypervisor. HV parameters are the ones that describe the virtualization-specific aspects of the instance; for example, what kernel to use to boot the instance (if any), or what emulation model to use for the emulated hard drives.
HVM
  Hardware virtualization mode, where the virtual machine is oblivious to the fact that’s being virtualized and all the hardware is emulated.
LogicalUnit
  The code associated with an OpCode, e.g. the code that implements the startup of an instance.
LUXI
  Local UniX Interface. The IPC method over unix(7) sockets used between the CLI tools/RAPI daemon and the master daemon.
OOB
  Out of Band. This term describes methods of accessing a machine (or parts of a machine) not via the usual network connection. For example, accessing a remote server via a physical serial console or via a virtual one IPMI counts as out of band access.
OpCode
  A data structure encapsulating a basic cluster operation; for example, start instance, add instance, etc.
PVM
  (Xen) Para-virtualization mode, where the virtual machine knows it’s being virtualized and as such there is no need for hardware emulation or virtualization.
SoR
  State of Record. Refers to values/properties that come from an authoritative configuration source. For example, the maximum VCPU over-subscription ratio is a SoR value, but the current over-subscription ration (based on how many instances live on the node) is a SoW value.
SoW
  State of the World. Refers to values that describe directly the world, as opposed to values that come from the configuration. Contrast with SoR.
tmem
  Xen Transcendent Memory (http://en.wikipedia.org/wiki/Transcendent_memory). It is a mechanism used by Xen to provide memory over-subscription.
watcher
  ganeti-watcher is a tool that should be run regularly from cron and takes care of restarting failed instances, restarting secondary DRBD devices, etc. For more details, see the man page ganeti-watcher(8).

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