HP StorageWorks SAN Virtualization Services Platform Best Practices Guide (5697-0935, May 2011)

9 Data migration
Migration tasks
Migration is a task, and counts against a domain limit of simultaneous tasks such as asynchronous
mirrors, snapclones, and remote copies. Because migration tasks immediately start processing after
the task has been created, and the priority of the migration task against the host I/Os cannot be
defined, HP recommends planning a migration when the application I/O requirements are lowest.
Figure 15 (page 37) shows how a migration is accomplished.
Figure 15 The migration process
A migration group is required to perform a migration. To create a migration group, create a
snapshot PiT on the source virtual disk or on each virtual disk group member. After the PiT creation,
the application writes are redirected to the temporary virtual disk created for the PiT.
PiTs and snapshots
Low capacity snapshots are common in the IT world. They provide a frozen image of the application
virtual disk‘s data at a particular point in time without having to occupy as much capacity as the
source. Low capacity snapshots are a key element in minimizing the backup window of applications.
SVSP has split the functionality of low capacity snapshots into two components:
• A point-in-time (PiT), which preserves the source data at the time the PiT is triggered. You can
have multiple PiTs based on the same source virtual disk. PiTs are only internal to SVSP and
cannot be mounted on hosts.
• A snapshot, which is a representation of a PiT that can be mounted, and has full read/write
capability. Writing to the snapshot does not overwrite the underlying PiT.
This combination of PiT and snapshot is essentially a "snapshot of a snapshot" in traditional terms.
Redirect-on-write technology that is used for PiTs and snapshots is a key function of SVSP.
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