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.

Image

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

Volume/Snapshot

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.