HP StorageWorks SAN Virtualization Services Platform Manager user guide (AG781-96017, March 2009)

14 Using snapshots
The snapshot service is a facility for making low-capacity snapshots. Snapshots can be made available
to any host for purposes such as testing or backup, while production data continues to be updated
without interruption.
The snapshot service enables you to create two types of entities:
• Point-in-Time (PiT)—An image of a Virtualization Services Manager (VSM) storage element, such
as a virtual disk. When you create a PiT on a virtual disk, VSM freezes the data in the virtual disk
and creates a new virtual disk, called a temporary virtual disk. Any subsequent modifications
made by the host application that uses the virtual disk are redirected to the temporary virtual disk.
• Snapshot—An entity that makes PiT data available to any host for any purpose. A snapshot initially
contains the data of the virtual disk at the time of PiT creation. You can grant read/write permission
to a host for the snapshot. The snapshot is exposed to the host as a virtual drive, just as a virtual
disk can be. The host’s modifications to the snapshot are directed to a temporary virtual disk that
VSM creates for the snapshot. Multiple snapshots can be created for the same PiT and be
presented to different hosts.
A snapshot temporary virtual disk contains a reallocation table that points to all chunks of data on
the original virtual disk that are modified by the host application after PiT creation or snapshot creation.
All modified chunks are replicated on the temporary virtual disk with their modifications. Chunks that
are not modified are not replicated on the temporary virtual disk. The temporary virtual disk grows
in size according to the number of chunks that are modified and the chunk size. The chunk size is
called the PiT granularity. It is set to large I/Os (1 MB) and is not configurable.
The snapshot service enables you to create multiple PiTs on one VSM storage element and to create
multiple snapshots of one PiT. A snapshot of a PiT created on a virtual disk group (VDG) is called a
VDG snapshot. PiT creation is supported on virtual disks, VDGs, snapshots, and VDG snapshots.
When you create a PiT on a VDG or VDG snapshot, a PiT is created on each member of the VDG or
VDG snapshot.
NOTE:
VDGs are encapsulation entities that you can create in order to perform synchronous operations on
multiple virtual disks or snapshots. During PiT creation for VDGs, the DPM fabric agent suspends new
write I/Os for VDG members, completes active I/O writes for all VDG members, and only then applies
the PiT creation synchronously by starting to use the temporary virtual disks for all VDG members
simultaneously. This action creates a crash consistent copy of the members of the VDG. For information
about creating VDGs and adding virtual disks and snapshots to VDGs, see
“Working with virtual disk groups” on page 125.
The snapshot service provides the following data recovery features:
• Rollback—Replaces virtual disk data with the data of a specified PiT. You can use this to recover
data after the corruption of a virtual disk or database.
• Restore—Deletes a virtual disk and replaces the virtual disk with a snapshot of the virtual disk.
This is useful in a testing environment.
Snapshots are a powerful tool that can be used for:
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