3.5.4. Host Storage

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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 be 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.
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 a guest to use storage area network (SAN) or iSCSI LUNs, as well as local CD-ROM media, 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 6.5 and above includes native support for creating virtual machines with 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, refer to the Red Hat Enterprise Linux 6 Virtualization Administration Guide.
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