3.5. Storage

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Storage for virtual machines is abstracted from the physical storage allocated to the virtual machine. It is attached to the virtual machine using the paravirtualized or emulated block device drivers.

3.5.1. Storage Pools

A storage pool is a file, directory, or storage device managed by libvirt for the purpose of providing storage to virtual machines. Storage pools are divided into storage volumes that store virtual machine images or are attached to virtual machines as additional storage. Multiple guests can share the same storage pool, allowing for better allocation of storage resources. For more information, see the Red Hat Enterprise Linux 7 Virtualization Deployment and Administration Guide.
Local storage pools
Local storage pools are attached directly to the host server. They include local directories, directly attached disks, physical partitions, and Logical Volume Management (LVM) volume groups on local devices. Local storage pools are useful for development, testing and small deployments that do not require migration or large numbers of virtual machines. Local storage pools may not be suitable for many production environment, because they do not support live migration.
Networked (shared) storage pools
Networked storage pools include storage devices shared over a network using standard protocols. Networked storage is required when migrating virtual machines between hosts with virt-manager, but is optional when migrating with virsh. Networked storage pools are managed by libvirt.

3.5.2. Storage Volumes

Storage pools are divided into storage volumes. Storage volumes are an abstraction of physical partitions, LVM logical volumes, file-based disk images and other storage types handled by libvirt. Storage volumes are presented to virtual machines as local storage devices regardless of the underlying hardware.

3.5.3. Emulated Storage Devices

Virtual machines can be presented with a range of storage devices that are emulated by the host. Each type of storage device is appropriate for specific use cases, allowing for maximum flexibility and compatibility with guest operating systems.
virtio-scsi is the recommended paravirtualized device for guests using large numbers of disks or advanced storage features such as TRIM. Guest driver installation may be necessary on guests using operating systems other than Red Hat Enterprise Linux 7.
virtio-blk is a paravirtualized storage device suitable for exposing image files to guests. virtio-blk can provide the best disk I/O performance for virtual machines, but has fewer features than virtio-scsi.
IDE is recommended for legacy guests that do not support virtio drivers. IDE performance is lower than virtio-scsi or virtio-blk, but it is widely compatible with different systems.
ATAPI CD-ROMs and virtio-scsi CD-ROMs are available and make it possible for guests to use ISO files or the host's CD-ROM drive. virtio-scsi CD-ROMs can be used with guests that have the virtio-scsi driver installed. ATAPI CD-ROMs offer wider compatibility but lower performance.
USB mass storage devices and floppy disks
Emulated USB mass storage devices and floppy disks are available when removable media are required. USB mass storage devices are preferable to floppy disks due to their larger capacity.

3.5.4. Host Storage

Disk images can be stored on a range of local and remote storage technologies connected to the host.
Image files
Image files can only be stored on a host file system. The image files can be stored on a local file system, such as ext4 or xfs, or a network file system, such as NFS.
Tools such as libguestfs can manage, back up, and monitor files. Disk image formats on KVM include:
Raw image files contain the contents of the disk with no additional metadata.
Raw files can either be pre-allocated or sparse, if the host file system allows it. Sparse files allocate host disk space on demand, and are therefore a form of thin provisioning. Pre-allocated files are fully provisioned but have higher performance than sparse files.
Raw files are desirable when disk I/O performance is critical and transferring the image file over a network is rarely necessary.
qcow2 image files offer a number of advanced disk image features, including backing files, snapshots, compression, and encryption. They can be used to instantiate virtual machines from template images.
qcow2 files are typically more efficient to transfer over a network, because only sectors written by the virtual machine are allocated in the image.
Red Hat Enterprise Linux 7 supports the qcow2 version 3 image file format.
LVM volumes
Logical volumes (LVs) can be used for disk images and managed using the system's LVM tools. LVM offers higher performance than file systems because of its simpler block storage model.
LVM thin provisioning offers snapshots and efficient space usage for LVM volumes, and can be used as an alternative to migrating to qcow2.
Host devices
Host devices such as physical CD-ROMs, raw disks, and logical unit numbers (LUNs) can be presented to the guest. This enables SAN or iSCSI LUNs as well as local CD-ROM media to be used by the guest with good performance.
Host devices can be used when storage management is done on a SAN instead of on hosts.
Distributed storage systems
Gluster volumes can be used as disk images. This enables high-performance clustered storage over the network.
Red Hat Enterprise Linux 7 includes native support for disk images on GlusterFS. This enables a KVM host to boot virtual machine images from GlusterFS volumes, and to use images from a GlusterFS volume as data disks for virtual machines. When compared to GlusterFS FUSE, the native support in KVM delivers higher performance.
For more information on storage and virtualization, see the Managing Storage for Virtual Machines.
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