Administrator Guide

Thin support in metro node
This chapter describes how metro node supports the thin-aware functionalities.
Topics:
• Thin support in metro node
• Thin provisioning
• Thin storage management
• Thin mirroring and migration
Thin support in metro node
Thin-aware is the functionality of advertising metro node virtual volumes as thin volumes to hosts. Thin volumes offer more
efficiency because the amount of resources used is much smaller than allocated. This benefit of providing only the resource
needed exceeds the cost of the virtualization technology that is used. It enables dynamic freeing of storage blocks on storage
volumes that have thin support. Thin support enables the mapping of one or more logical blocks to physical blocks, when
required. The logical blocks provide the storage address space (logical unit capacity) to hosts. Physical storage is only allocated
to the logical unit when it is used. This ensures that the logical unit is allocated less physical storage than it reports as its
capacity. The physical blocks can be mapped to the logical blocks when required (on write). Metro node extends multiple thin
capabilities that are provided by the arrays that are attached to the back-end.
Thin storage management
Metro node uses some of the management capabilities of the thin-capable arrays in its back-end to detect and address the
storage exhaustion issues. When a host stops using the allocated thin storage blocks from the array, the unused blocks are
not freed up and they are not returned to the arrays. For example, in a virtual environment where the data in datastores of a
virtual machine are stored on a thin volume, and these datastores are deleted or moved, the storage space is not freed up. This
behavior can result in an out-of-space issue on the thin volumes. When the thin storage capacity reaches a specific threshold,
the storage arrays send out events to the hosts indicating that the storage space is diminishing. In such cases, the hosts can
send the SCSI UNMAP command to the metro node virtual volumes to free up the unused space.
NOTE:
The UNMAP feature is supported only on the thin-enabled metro node virtual volumes that meet the thin
requirements. Creating thin-enabled virtual volumes lists the thin requirements for a virtual volume.
Thin rebuild
Metro node provides continuous availability and high availability functionalities through its mirroring capability. During the
mirroring process, metro node ensures that a thin mirror leg does not turn into a thick leg. Metro node uses its thin rebuild
feature to synchronize the data between the mirrors of a RAID-1 device that is built on thin volumes. If the array supports
the UNMAP feature, metro node uses the SCSI UNMAP commands to free up space on out of date legs if applicable. If the
array does not support the UNMAP feature, metro node writes zeros to blocks that have to be zeroed to preserve thinness.
This behavior allows preserving the thinness of the device. Even before the UNMAP support, metro node allowed a metro node
administrator to claim a thin storage volume by setting the thin-rebuild flag. It directs metro node to make efficient use of
the space using thin rebuilds.
Rebuilds of thin provisioned storage provides you more information on the thin provisioned storage rebuilds.
Thin migrations
Metro node supports data mobility features on thin devices. When the migration source or the target is not thin, or the source
and the targets are from dissimilar storage-array families, the metro node virtual volume loses its thin properties. In such a case,
the virtual volume does not support thin storage management operations. After the migration is completed and committed, the
5
Thin support in metro node 27