HP 3PAR StoreServ Concepts Guide: HP 3PAR OS 3.1.3

• A maximum of 500 virtual copies can be made from one base volume.
• A virtual volume cannot be deleted if a child copy of it exists. For example, S1 cannot be
removed unless S1_0, S1_0_0, and S1_0_1 are deleted first.
Copy-of and Parent Relationships
In the example in Figure 11 (page 49), there are two different tree structures: the solid arrows
show the copy-of relationships, and the dashed arrows show the parent relationship. For example,
S0 is a read-only copy of BaseVV, and S1 is the parent of S0. The copy-of relationship shows 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 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 that are needed to
reconstruct the earlier state of the virtual volume. The farther 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 LUN.
The characteristics of this association are defined when you create a 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 HP SPOCK website:
http://www.hp.com/storage/spock
Virtual volumes can be exported with the HP 3PAR CLI and the HP 3PAR Management Console.
See the HP 3PAR Command Line Interface Administrator’s Manual and the HP 3PAR Management
Console Online Help for instructions on performing 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.
After 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 stops 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
50 Virtual Volumes