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52 Planning: MD3600i Series Storage Array Terms and Concepts
Snapshot Virtual Disks
A snapshot is a point-in-time image of a virtual disk. The snapshot provides
an image of the virtual disk at the time the snapshot was created. You create a
snapshot so that an application (for example, a backup application) can
access the snapshot and read the data while the source virtual disk remains
online and user-accessible. When the backup is completed, the snapshot
virtual disk is no longer needed. You can create up to four snapshots per
virtual disk.
Snapshots are used to recover previous versions of files that have changed
since the snapshot was taken. Snapshots are implemented using a copy on
write algorithm, which makes a backup copy of data the instant a write occurs
to the virtual disk. Data on a virtual disk is copied to the snapshot repository
before it is modified. Snapshots are instantaneous and take up less overhead
than a full physical copy process.
Snapshot Repository Virtual Disk
When you create a snapshot virtual disk, it automatically creates a snapshot
repository virtual disk. A snapshot repository is a virtual disk created in the
storage array as a resource for a snapshot virtual disk. A snapshot repository
virtual disk contains snapshot virtual disk metadata and copy-on-write data
for a particular snapshot virtual disk. The repository supports one snapshot
only.
You cannot select a snapshot repository virtual disk as a source virtual disk or
as a target virtual disk in a virtual disk copy. If you select a Snapshot source
virtual disk as the target virtual disk of a virtual disk copy, you must disable all
snapshot virtual disks associated with the source virtual disk.
CAUTION: Before using the Snapshot Virtual Disks Premium Feature in a
Windows Clustered configuration, you must map the snapshot virtual disk to the
cluster node that owns the source virtual disk. This ensures that the cluster nodes
correctly recognize the snapshot virtual disk.
Mapping the snapshot virtual disk to the node that does not own the source
virtual disk before the snapshot enabling process is completed can result in
the operating system misidentifying the snapshot virtual disk. This can result
in data loss or an inaccessible snapshot.
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