3.2. The physical and logical size of an LVM-VDO volume
This section describes the physical size, available physical size, and logical size that VDO can utilize.
- Physical size
This is the same size as the physical extents allocated to the VDO pool LV. VDO uses this storage for:
- User data, which might be deduplicated and compressed
- VDO metadata, such as the UDS index
- Available physical size
This is the portion of the physical size that VDO is able to use for user data.
It is equivalent to the physical size minus the size of the metadata, rounded down to a multiple of the slab size.
- Logical Size
This is the provisioned size that the VDO LV presents to applications. It is usually larger than the available physical size. VDO currently supports any logical size up to 254 times the size of the physical volume with an absolute maximum logical size of 4 PB.
When you set up a VDO logical volume (LV), you specify the amount of logical storage that the VDO LV presents. When hosting active VMs or containers, Red Hat recommends provisioning storage at a 10:1 logical to physical ratio, that is, if you are utilizing 1 TB of physical storage, you would present it as 10 TB of logical storage.
If you do not specify the
--virtualsizeoption, VDO provisions the volume to a1:1ratio. For example, if you put a VDO LV on top of a 20 GB VDO pool LV, VDO reserves 2.5 GB for the UDS index, if the default index size is used. The remaining 17.5 GB is provided for the VDO metadata and user data. As a result, the available storage to consume is not more than 17.5 GB, and can be less due to metadata that makes up the actual VDO volume.
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