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3.3. Storage Attributes

The following attributes are given to storage objects either by the Red Hat Virtualization Manager or by a system administrator using the Red Hat Enterprise Virtualization Manager.

3.3.1. Storage Format Types

The Red Hat Enterprise Virtualization environment supports two storage formats: RAW and QCOW2.

3.3.1.1. QCOW2

QCOW stands for QEMU copy on write. The QCOW2 format decouples the physical storage layer from the virtual layer by adding a mapping between logical and physical blocks. Each logical block is mapped to its physical offset. This mapping enables advanced features like snapshots. Creating a new snapshot creates a new copy on write layer, either a new file or logical volume, with an initial mapping that points all logical blocks to the offsets in the backing file or volume. When writing to a QCOW2 volume, the relevant block is read from the backing volume, modified with the new information and written into the new snapshot QCOW2 volume. Then the map is updated to point to the new place. Benefits QCOW2 offers over using RAW representation include:
  • Copy-on-write support, where a volume only represents changes made to an underlying disk image.
  • Snapshot support, where a volume can represent multiple snapshots of the images history.

3.3.1.2. RAW

The RAW storage format has a performance advantage over QCOW2 in that no formatting is applied to images stored in the RAW format. Reading and writing images stored in RAW format requires no additional work on the part of the host or Manager. When the guest file system writes to a given offset in its virtual disk, the I/O will be written to the same offset on the backing file or logical volume. Raw format requires that the entire space of the defined image be preallocated unless using externally managed thin provisioned LUNs from a storage array.