Why a backup takes the full provisioned size of the thin disk rather than used size ?
Applies to
- ONTAP 9
- Veritas NBU
- NFS
- VMware vSphere
Answer
There is a VMware limitation about non-VMFS datastores when using thin provisioned disks.
When performing a backup of a virtual machine where thin provisioning is used, one of the steps is to query vSphere to find out which blocks are in use. This is done via a QueryChangeDiskAreas("*") call. The information below, copied from VMware documentation, explains the behavior of this query command when thin provisioned virtual disks are involved.
So, it will perform the backup of vmdk's located on non-VMFS datastores like for example NFS, the full provisioned size of the thin disks will be sent to the server.
When performing a backup of a virtual machine where thin provisioning is used, one of the steps is to query vSphere to find out which blocks are in use. This is done via a QueryChangeDiskAreas("*") call. The information below, copied from VMware documentation, explains the behavior of this query command when thin provisioned virtual disks are involved.
"About Changed Block Tracking :
QueryChangedDiskAreas("*") returns information about areas of a virtual disk that are in use (allocated). The current implementation depends on VMFS properties, similar to properties that SAN transport mode uses to locate data on a SCSI LUN.
Both rely on unallocated areas (file holes) in virtual disk, and the LazyZero designation for VMFS blocks. Thus, changed block tracking yields meaningful results only on VMFS.
On other storage types, it either fails, or returns a single extent covering the entire disk.
You should enable changed block tracking in the order recommended by Enabling Changed Block Tracking.
The first time you call QueryChangedDiskAreas("*"), it should return allocated areas of virtual disk. Subsequent calls return changed areas, instead of allocated areas. If you call QueryChangedDiskAreas after a snapshot but before you enable changed block tracking, it also returns unallocated areas of virtual disk.
With thin provisioned virtual disk this could be a large amount of zero data.
The guest operating system has no visibility of changed block tracking. Once a virtual machine has written to a block on virtual disk, the block is considered in use.
The information required for the "*" query is computed when changed block tracking is enabled,
and the .ctk file is pre-filled with allocated blocks.
The mechanism cannot report changes made to virtual disk before changed block tracking was enabled."So, it will perform the backup of vmdk's located on non-VMFS datastores like for example NFS, the full provisioned size of the thin disks will be sent to the server.