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2.3. Storage

Red Hat Enterprise Virtualization uses a centralized storage system for virtual machine disk images, templates, snapshots, and ISO files. Storage is logically grouped into storage pools, which are comprised of storage domains. A storage domain combines storage capacity that contains images with metadata that describes the internal structure of the storage. There are three types of storage domain; data, export, and ISO.
The data storage domain is the most important and is the only one required by each data center. The data storage domain is exclusive to a single data center. Export and ISO domains are optional. Storage domains are shared resources, and must be accessible to all hosts in a data center. Storage networking can be implemented using Network File System (NFS), Internet Small Computer System Interface (iSCSI), or Fibre Channel Protocol (FCP). A storage domain can consist of block devices (SAN - iSCSI or FCP) or files (NAS - NFS).
On NFS, all virtual disks, templates, and snapshots are simple files. On SAN (iSCSI/FCP), block devices are aggregated into a logical entity called a Volume Group (VG). This is done using the Logical Volume Manager (LVM). See Red Hat Enterprise Linux Logical Volume Manager Administration Guide for more information on LVM. Each virtual disk, template or snapshot is a Logical Volume (LV) on the VG.
Storage Architecture
Figure 2.3. Storage Architecture

Data storage domain
Data domains hold the disk images of all the virtual machines running in the environment, including installed operating system images, and data disks. Snapshots of the virtual machines are also stored in the data domain. A data domain cannot be shared across data centers, and the data domain must be of the same type as the data center. For example, a data center of a iSCSI type, must have an iSCSI data domain.
Export storage domain
An export domain is a temporary storage repository that is used to copy and move images between data centers and Red Hat Enterprise Virtualization environments. The export domain can be used to back up virtual machines and templates. An Export domain can be moved between data centers, but can only be active in one data center at a time.
ISO storage domain
ISO domains store ISO files, which are logical CD-ROMs used to install operating systems and applications for the virtual machines. As a logical entity that replaces a library of physical CD-ROMs or DVDs, an ISO domain removes the data center's need for physical media. An ISO domain can be shared across different data centers.
See Chapter 3, Storage Architecture for more information on the Red Hat Enterprise Virtualization storage architecture.