HP 3PAR StoreServ 7400 2-node HP 3PAR StoreServ Storage Concepts Guide (OS 3.1 - Page 48

• Each copy tracks changes made to BaseVV from its own creation date until the next snapshot is made. • S1_0 can be created at any time after S1 is created. The relationships between the virtual copies derived from a base volume can be represented as a tree. In the example in Figure 9 (page 47), the base volume BaseVV is the starting point. In this example, each new virtual copy of the original has its name incremented by 1. Each copy of a copy has an additional level added to its name: in this example, the first copy of S1 is S1_0, and a copy of S1_0 is S1_0_0. Unlike the automatic snapshots created for physical copies, these snapshots are not assigned names by the system. NOTE: The naming convention used in the example above is recommended, but it is not enforced by the system. You can name each virtual volume and virtual copy at the time of creation. The following rules are enforced by the system when you create a snapshot: • The tree grows in alternating layers of read/write and read-only snapshots. You can only make a read-only copy of a read/write volume, and you can only make a read/write copy of a read-only volume. • A maximum of 256 read/write virtual copies can be made from one read-only virtual volume. • 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 9 (page 47), 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 simply 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 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://www.hp.com/storage/spock. 48 Virtual Volumes