Thin provisioning

Thin provisioning defines logical volume sizes that are larger than the usable capacity installed on the system. The volume allocates capacity on an as-needed basis as a result of host-write actions.

The thin provisioning feature enables the creation of extent space efficient logical volumes. Extent space efficient volumes are supported for FB and CKD volumes and are supported for all Copy Services functionality, including FlashCopy targets where they provide a space efficient FlashCopy capability.

Releasing space on CKD volumes that use thin provisioning

On an IBM Z® host, the DFSMSdss SPACEREL utility can release space from thin provisioned CKD volumes. The behavior depends on the Copy Services relationships that exist on the volume at the time the space release command is received.

Table 1. Space Release support for Copy Services volumes

Copy Services type	State	Space release allowed	Space release behavior
Metro Mirror	Duplex	Yes	Space is released on the primary and secondary. If the secondary copy is the primary copy of another Global Copy or Global Mirror relationship, space is also released on secondary copies of that relationship.
	Pending	Yes	Space is released on the primary and secondary. If the secondary copy is the primary copy of another Global Copy or Global Mirror relationship, space is also released on secondary copies of that relationship.
	Suspended	Yes	Space is released on the primary.
Global Mirror	Pending	Yes	Space is released on the primary copy after a new consistency group is formed. Space is released on the secondary copy after the next consistency group is formed and a FlashCopy commit is performed. If the secondary copy is the primary copy of another Global Mirror or Global Copy relationship, space is also released on secondary copies of that relationship.
	Suspended	Yes	Space is released on the primary.
Global Copy	Pending	Yes	Space is released on the primary and secondary copies. If the secondary copy is the primary copy of another Global Copy relationship, space is also released on secondary copies of that relationship.
	Suspended	Yes	Space is released on the primary.
z/OS Global Mirror	Primary	No	n/a
FlashCopy	Source	No	n/a
	Target	No	n/a
Safeguarded Copy	Source	Yes	Space is released when the Safeguarded backups prior to the space release command have been rolled off.
Safeguarded Recovery	Source	No	n/a
	Target	No	n/a

Space is released on Copy Services target volumes when these relationships are created or deleted:

• Space is released on FlashCopy targets when the FlashCopy is established or withdrawn.
• Space is released on Metro Mirror, Global Copy, and Global Mirror targets when the initial copy is performed.

Space is also released on a regular basis for Global Mirror FlashCopy targets as part of the consistency group formation process. To pace this process and avoid excessive release and reallocation, this is controlled by an internal algorithm, rather than releasing space every time a FlashCopy is performed.