HP 3PAR InForm OS 3.1.1 Concepts Guide

that the snapshot was created by copying another virtual volume. The parent relationship refers to
the internal organization of the administration space. The parent volume contains information
needed to reconstruct the snapshot represented by the child volume. A parent volume can have a
creation date after that of its child if the parent volume was modified.
The parent relationship is useful for two reasons:
• Understanding the performance consequences of virtual copies. The tree representing the
parent relationship shows the look-up paths in the administration space needed to reconstruct
the earlier state of the virtual volume. The farther away a virtual copy is from the base volume,
the longer it will take to retrieve it. If a snapshot is expected to be kept in use for a long time,
consider making a physical copy instead of a virtual copy.
• Understanding which virtual copies become stale if the administration space is full and the
copy-on-write data cannot be written. A stale snapshot is one that cannot be completely
recreated because the most recent changes will not be included. The current snapshot and all
its children become stale when a write fails. For example, if there is no space to write the
copy-on-write data when a host writes to S1_0, then S1_0, S1_0_1, and S1_0_0 become stale.
Exporting Virtual Volumes
Virtual volumes are the only data layer component visible to hosts. You export a virtual volume to
make it available to one or more hosts by creating an association between the volume and a
logical unit number (LUN). The characteristics of this association are defined when you create a
Virtual Volume-LUN pairing (VLUN). A VLUN is a pairing between a virtual volume and a LUN
expressed as either a VLUN template or an active VLUN. For the maximum number of VLUNs
supported for each host with your specific system configuration, go to the Single Point of Connectivity
Knowledge (SPOCK) website http://spock.corp.hp.com/index.aspx.
Exporting virtual volumes can be performed with both the HP 3PAR InForm Command Line Interface
(CLI) and the HP 3PAR InForm Management Console. Refer to the HP 3PAR InForm OS CLI
Administrator’s Manual and the HP 3PAR InForm OS Management Console Online Help for
instructions on how to perform this task.
VLUN Templates and Active VLUNs
A VLUN template sets up an association between a virtual volume and a LUN-host, LUN-port, or
LUN-host-port combination by establishing the export rule. When you create a VLUN template, if
the current system state meets the conditions established by the VLUN template, that template is
immediately applied to create one or more active VLUNs. These active VLUNs enable virtual
volumes to be exported to hosts. If the current system state does not meet the conditions of the
VLUN template, no active VLUNs are created until the conditions of the template are met.
Once a VLUN template is applied to create one or more active VLUNs, hosts continue to be able
to access volumes based on the export rule established by that template. Removing VLUNs associated
with a volume halts host access to that volume. Removing all VLUNs for a host stops the host from
accessing all volumes.
VLUN Template Types
A VLUN template sets up an association between a virtual volume and a LUN-host, LUN-port, or
LUN-host-port combination by establishing the export rule, or the manner in which the volume is
exported. A VLUN template enables the export of a virtual volume as a VLUN to a host or hosts.
Those volume exports, which are seen as LUNs by the host or hosts, are active VLUNs.
A VLUN template can be one of the following types:
• Host sees allows only a specific host to see a volume.
• Host set allows any host that is a member of the host set to see a volume.
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