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Content Owner: Roman Macek | ||||
Summary
RAW, Qcow2
Details
RHV supports two storage formats: RAW and QCOW2.
In an NFS data center the Storage Pool manager (SPM) creates the virtual disk on top of a regular file system as a normal disk in preallocated (RAW) format. Where sparse allocation is chosen additional layers on the disk will be created in thinly provisioned Qcow2 (sparse) format.
For iSCSI and SAN (block), the SPM creates a Volume group (VG) on top of the Logical Unit Numbers (LUNs) provided. During the virtual disk creation, either a preallocated format (RAW) or a thinly provisioned Qcow2 (sparse) format is created.
Background:
QCOW (QEMU copy on write) decouples the physical storage layer from the virtual layer by adding a mapping between logical and physical blocks. This 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. T hen the map is updated to point to the new place.
Benefits QCOW2 offers over using RAW representation include:
- Copy-on-write support (volume only represents changes to a disk image).
- Snapshot support (volume can represent multiple snapshots of the images history).
The RAW storage format has a performance advantage over QCOW2 as no formatting is applied to images stored in the RAW format (reading and writing images stored in RAW requires no additional mapping or reformatting work on 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. Note: Raw format requires that the entire space of the defined image be preallocated (unless using externally managed thin provisioned LUNs from a storage array).
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-IO intensive virtual machines.
In an NFS data center the Storage Pool manager (SPM) creates the virtual disk on top of a regular file system as a normal disk in preallocated (RAW) format. Where sparse allocation is chosen additional layers on the disk will be created in thinly provisioned Qcow2 (sparse) format.
For iSCSI and SAN (block), the SPM creates a Volume group (VG) on top of the Logical Unit Numbers (LUNs) provided. During the virtual disk creation, either a preallocated format (RAW) or a thinly provisioned Qcow2 (sparse) format is created.
Background:
QCOW (QEMU copy on write) decouples the physical storage layer from the virtual layer by adding a mapping between logical and physical blocks. This 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. T hen the map is updated to point to the new place.
Benefits QCOW2 offers over using RAW representation include:
- Copy-on-write support (volume only represents changes to a disk image).
- Snapshot support (volume can represent multiple snapshots of the images history).
The RAW storage format has a performance advantage over QCOW2 as no formatting is applied to images stored in the RAW format (reading and writing images stored in RAW requires no additional mapping or reformatting work on 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. Note: Raw format requires that the entire space of the defined image be preallocated (unless using externally managed thin provisioned LUNs from a storage array).
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-IO intensive virtual machines.
