Chapter 10. Virtual Machine Disks

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10.1. Understanding Virtual Machine Storage

Red Hat Enterprise Virtualization supports three storage types: NFS, iSCSI and FCP.
In each type, a host known as the Storage Pool Manager (SPM) manages access between hosts and storage. The SPM host is the only node that has full access within the storage pool; the SPM can modify the storage domain metadata, and the pool's metadata. All other hosts can only access virtual machine hard disk image data.
By default in an NFS, local, or POSIX compliant data center, the SPM creates the virtual disk using a thin provisioned format as a file in a file system.
In iSCSI and other block-based data centers, the SPM creates a volume group on top of the Logical Unit Numbers (LUNs) provided, and makes logical volumes to use as virtual machine disks. Virtual machine disks on block-based storage are preallocated by default.
If the virtual disk is preallocated, a logical volume of the specified size in GB is created. The virtual machine can be mounted on a Red Hat Enterprise Linux server using kpartx, vgscan, vgchange or mount to investigate the virtual machine's processes or problems.
If the virtual disk is thinly provisioned, a 1 GB logical volume is created. The logical volume is continuously monitored by the host on which the virtual machine is running. As soon as the usage nears a threshold the host notifies the SPM, and the SPM extends the logical volume by 1 GB. The host is responsible for resuming the virtual machine after the logical volume has been extended. If the virtual machine goes into a paused state it means that the SPM could not extend the disk in time. This occurs if the SPM is too busy or if there is not enough storage space.
A virtual disk with a preallocated (RAW) format has significantly faster write speeds than a virtual disk with a thin provisioning (QCOW2) format. Thin provisioning takes significantly less time to create a virtual disk. The thin provision format is suitable for non-I/O intensive virtual machines. The preallocated format is recommended for virtual machines with high I/O writes. If a virtual machine is able to write more than 1 GB every four seconds, use preallocated disks where possible.
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