Openvirt

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Storage Support
	Local Storage and SAN
	Shared Storage Filesystems
	Thin Provisioning
	Disk I/O Performance

Local Storage and SAN

The platform offers a wide warranty of options to provide storage for the virtual machines. If it works with Linux, it works with Openvirt.

Local Storage
A fixed disk, RAID array or DAS for your virtual machines back-end storage. Restricts access to the virtual machines to only one host. Live migration is not possible. However, local storage is still very useful for test environments and for evaluation of the platform itself.

SAN Based Storage
iSCSI over Ethernet, ATA over Ethernet, Fibre Channel or SCSI over native InfiniBand for very high performance. The best option when using multiple hosts.

NAS Based Storage
NFS as a shared storage system between different hosts. Useful for test environments.

Shared Storage Filesystems

For SAN based setups, you can use a shared storage filesystem. This dramatically simplifies the back-end storage when using multiple hosts. Provisioning new machines is very simple when using virtual disks which are file based block devices. Live migration is also greatly simplified.

For shared storage filesystems we offer 2 choices: Oracle's OCFS2 and Red Hat's GFS. Both are POSIX-compliant shared-disk cluster file systems capable of providing both high performance and high availability. As they provide local file system semantics, they can be used with any application.

Thin Provisioning

With the file based virtual disks approach, you can benefit from the thin provisioning feature.

What is thin provisioning? When virtual disks are created, the total size of the virtual disk is not fully allocated on the underlying filesystem. Rather storage is only allocated when it needs to be. This happens without any configuration in the the guest operating system.

Although there is some benefit of being able to overcommit your total storage, the real benefit comes from the ability to create much smaller backups of the virtual disks. This is very desirable when doing scheduled daily backups of your virtualization infrastructure.

The table below shows some real world examples of file based virtual disks that contain the complete operating system for a virtual machine.

From top to bottom: Debian 5.0, Red Hat Enterprise Linux 5.3, Ubuntu 9.04 Server and Windows Server 2008 R2 Enterprise. For the backups, bzip2 is used for compression.

Disk I/O Performance

One of KVMs strengths is disk and network I/O. These are the primary concerns for system administrators when looking to migrate physical infrastructure to virtual infrastructure.

KVM provides high performance virtual disk controller performance.

OCZ Vertex solid state drive. Test is file based on an NTFS filesystem.	QEMU-KVM virtual disk drive. Test is file based on an NTFS filesystem.
Physical and virtual compared. On the left is an OCZ Vertex solid state drive benchmarked on a physical system. On the right is a virtual hard disk on a virtual machine. Both are running Windows Server 2008. The filesystem is NTFS. Click to enlarge.