Dell EMC Metro node 7.0.1 Product Guide

Support for thin volumes and unmapping

Thin provisioning advertises the metro node virtual volumes as thin volumes to the hosts. Thin provisioning dynamically allocates block resources only when they are required. It essentially allows efficient utilization of physical block resources from the storage arrays.

Hosts gather the properties related to the thin provisioning feature of a metro node virtual volume and send SCSI commands to free storage block resources that are not in use. If the blocks of the back end storage volumes are free, the blocks can be mapped to other changed regions. Thin provisioning enables dynamic freeing of storage blocks on storage volumes for which thin provisioning is supported.

Metro node thin provisioning support includes the following features:

• Discovery of the back-end storage volumes capable for thin provisioning - During the back-end storage volume discovery, metro node gathers all thin provisioning related storage volume properties. Metro node also performs a consistency check on all the properties related to thin-provisioning.
• Reporting thin provisioning enabled metro node virtual volumes to hosts - metro node shares the details of the thin provisioning-enabled virtual volumes with the hosts.
• Reclaiming the unused storage blocks - Through a command, metro node removes the mapping between a deleted virtual machine and its storage volumes and reclaims the storage blocks corresponding to the VMFS blocks used by that virtual machine.
• Handling storage exhaustion - The exhaustion of storage blocks on non-mirrored storage volumes are notified to the host as a space allocation failure. This error notification is posted to the host and the VMware hosts stop the impacted virtual machine.

To prevent potential mapping of all the blocks in the storage volumes that are thin capable, metro node uses thin rebuilds. Thin rebuilds can be configured to be set or unset for any claimed storage volume on which metro node builds virtual volumes. This property controls how metro node does its mirror rebuilding.

The unmap feature reclaims the unused VMFS blocks by removing the mapping between the logical blocks and the physical blocks. This essentially removes the link between a logical block and a physical block that has unknown or unused resources.