HP StorageWorks SAN Virtualization Services Platform Manager User Guide (5697-0454, May 2010)

17 Using migration
The migration service is the facility that copies and moves the data within VSM virtual disks from one
storage pool to another. During migration, host applications remain online and continue to read and
write from the migrating virtual disk. The migration service keeps the data continuously online while
implementing Information Lifecycle Management (ILM) or while upgrading hardware. Host applications
using the migrating virtual disk are not disrupted.
The migration service features:
• Source virtual disk backup—By the end of the migration, the original virtual disk with its original
data remain on their original storage pool and can be use for backup or testing.
• Suspend and resume—You can suspend and resume migration tasks according to the organization’s
needs and network load.
To perform a migration, you first create a migration group. A migration group defines the virtual disk
or virtual disk group (VDG) that you want to migrate. When you create the group, you specify a
destination storage pool and an auxiliary name. Once the group is created, a migration process,
called a migration task, begins. When the task finishes, both the task and group are deleted. The
virtual disk or VDG is now located on back-end LUs in the destination storage pool, and a copy of
the virtual disk or VDG remains in the source storage pool for backup.
When you create a migration group, the migration service creates a snapshot PiT on the source virtual
disk or on each VDG member, if your source element is a VDG. After PiT creation, the application
writes are redirected to the temporary virtual disk created for the PiT. Application reads result in either
a read from the source virtual disk or from the PiT temporary virtual disk. The migration task copies
the data of the source virtual disk, which is now changing to a virtual disk named with the auxiliary
name you provided on the destination storage pool. When the task finishes copying the data to the
destination, the destination virtual disk contains the data of the source virtual disk at the time of the
PiT creation.
The VSM server then switches the virtual disk with which the PiT is associated. Originally, the PiT is
associated with the source virtual disk. After the switch, the PiT becomes associated with the auxiliary
virtual disk on the destination storage pool, which is, at that point, identical to the source virtual disk.
The names of the source virtual disk and auxiliary virtual disk are then swapped. The PiT is then
deleted and the PiT data is synchronized with the destination virtual disk, which now has the original
virtual disk name.
The result of a migration task is an up-to-date virtual disk on the destination storage pool. It has the
name of the original virtual disk, to which the host application reads and writes and a backup virtual
disk on the original storage pool with the auxiliary name you provided, and contains the data that
the virtual disk contained at the time you created the migration group.
Migration is typically implemented for the following purposes:
• Data center hardware upgrade—With the migration service, you can place new hardware into
new storage pools and migrate virtual disks to the new storage pools without disrupting applications
that are using the virtual disks. You can also expand the storage available for all virtual disks by
adding new hardware, creating new storage pools from the new hardware, migrating some of
the virtual disks to these new storage pools, and leaving some virtual disks on the old storage
pools.
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