Vdsm Storage Terminology

Storage Pool

A group of domains that are managed together. Currently domains grouped and can only be managed while being a part of a pool. We plan to remove the limitations created by using storage pools.

Storage Domain

An atomic storage unit. On file domains it’s either a mount point or a folder. On block devices on the other hand this can be a group of LUNs. There is no hard limitation but it is highly recommended that all the LUNs composing a block domain are on the same physical host. This is done because when only parts of the domain disappear in case of failure there is a real issue with detecting problems.

Storage domains contains the images that the VMs will use.

Storage domains split to 2 distinct groups.

  1. File Based Storage (NFS and Local FS targets)
    • pros:
      • Usually takes care of safe access to files across the cluster
      • Faster setup
    • cons:
      • Slower
  2. '’Block Based Storage (SAN Storage and in the future Local Disks)
    • pros:
      • Faster
    • cons:
      • VDSM has to do some extra work to make sure consistency
      • Slower setup

Vdsm tries to keep feature parity between the two major types. There are a few things to note:

  • Some operations may differ in performance between the two types.
  • RAW/SPARSE images are currently only supported on file domains. This feature is implemented using sparse files and this feature currently has no equivalent in the block/LVM world.


A group of one or more volumes comprising a disk image to be used by VMs.


Currently volumes and snapshots are the same. On block domain each volume is translated to an LV (logical volume) and on file domains to a separate file. Base volumes can be either raw or qcow but snapshots must be in qcow format.