HP StorageWorks 2000 Modular Smart Array Reference Guide (481599-003, August 2008)

Chapter 3 Managing Storage 121
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.
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.
Volume Type Field Description