Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

See “Veritas FlashSnap Agent for Symmetrix” on page 138.
Volume snapshots
A snapshot is a virtual image of the content of a set of data at the instant of
creation. Physically, a snapshot may be a full (complete bit-for-bit) copy of the
data set, or it may contain only those elements of the data set that have been
updated since snapshot creation. The latter are sometimes referred to as
allocate-on-first-write snapshots, because space for data elements is added to the
snapshot image only when the elements are updated (overwritten) for the first
time in the original data set. Storage Foundation allocate-on-first-write snapshots
are called space-optimized snapshots.
A full-copy snapshot of a virtual volume requires storage capacity equal to that
of the original volume, as well as sufficient I/O bandwidth to copy the entire
contents of the original volume to the snapshot within the required time.
Space-optimized snapshots, on the other hand, consume storage and I/O bandwidth
in proportion to how much data on the original volume is updated during a
snapshot’s life. Thus, full-copy snapshots require more storage and I/O bandwidth
than space-optimized ones. They are generally more suitable for recovering from
storage device failures, and for off-host auxiliary processing where impact on
production applications is a concern. They are generally less optimal for protecting
against data corruption due to human or application error.
Space-optimized snapshots do not contain complete physical images of the original
data objects they represent, and they cannot be taken off-host for auxiliary
processing. However, because they consume relatively little storage and I/O
bandwidth, they can be taken much more frequently than full-copy snapshots.
This makes them well-suited for recovering from data corruption. Space-optimized
snapshots naturally tend to grow with age, as more of the data in the original
objects changes, so they are inherently better-suited for shorter lifetimes.
Persistent FastResync of volume snapshots
Veritas Volume Manager allows you to take multiple snapshots of your data at
the level of a volume. A snapshot volume contains a stable copy of a volume’s data
at a given moment in time that you can use for online backup or decision support.
If persistent FastResync is enabled on a volume, VxVM uses a FastResync map to
keep track of which blocks are updated in the volume and in the snapshot. If the
data in one mirror is not updated for some reason, it becomes out-of-date, or stale,
with respect to the other mirrors in the volume. The presence of the FastResync
map means that only those updates that the mirror has missed need be reapplied
to resynchronize it with the volume. A full, and therefore much slower,
resynchronization of the mirror from the volume is unnecessary.
Understanding point-in-time copy methods
About point-in-time copy technology
134