HP StorageWorks MSA2012i HP StorageWorks 2000 Modular Smart Array Reference Gu - Page 121
Using Volume-Copy Services, Additional Data Protection, Non-disruptive Use of Production Data
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Volume Type Field Description Snap Data - The total amount of data associated with the specific snapshot (data copied from a master volume to a snapshot and data written directly to a snapshot). Unique Data - The amount of data that has been written to the snapshot since the last snapshot was taken. If the snapshot has not been written or is deleted, this field will show a value of 0. Shared Data - The amount of data that is potentially shared with other snapshots and associated amount of space that is guaranteed to be freed if the snapshot is deleted. This represents the amount of data written directly to the snapshot. It also includes data copied from the master volume to the snap pool for the oldest snapshot, since that snapshot does not share data with any other snapshot. For a snapshot that is not the oldest, if the modified data is deleted or if it had never been written to, this field will show a value of 0. Using Volume-Copy Services While a snapshot is a point-in-time logical copy of a volume, the volume-copy service creates a complete, physical and independent copy of a volume within a storage system. It is an exact copy of a master or a snapshot volume as it existed at the time the action was initiated, consumes the same amount of space as the source volume, and is independent from an I/O perspective. Volume independence is a key distinction of a volume copy (versus snapshot, which is a logical copy and dependent on the source volume). Benefits include: ■ Additional Data Protection - An independent copy of a volume (versus logical copy through snapshot) provides additional data protection against a complete master volume failure. If the source master volume fails, the volume copy can be used to restore the volume to the point in time the volume copy was taken. ■ Non-disruptive Use of Production Data - With an independent copy of the volume, resource contention and the potential performance impact on production volumes is mitigated. Data blocks between the source and the copied volumes are independent (versus shared with snapshot) so that I/O is to each set of blocks respectively; application I/O transactions are not competing with each other when accessing the same data blocks. Chapter 3 Managing Storage 121