Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

plexes from the original volume (which is similar to the way that the vxassist
command creates its snapshots).
Unlike a third-mirror break-off snapshot created using the vxassist command,
you can make a backup of a full-sized instant snapshot, instantly refresh its
contents from the original volume, or attach its plexes to the original volume,
without completely synchronizing the snapshot plexes from the original volume.
VxVM uses a copy-on-write mechanism to ensure that the snapshot volume
preserves the contents of the original volume at the time that the snapshot is
taken. Any time that the original contents of the volume are about to be
overwritten, the original data in the volume is moved to the snapshot volume
before the write proceeds. As time goes by, and the contents of the volume are
updated, its original contents are gradually relocated to the snapshot volume.
If a read request comes to the snapshot volume, yet the data resides on the original
volume (because it has not yet been changed), VxVM automatically and
transparently reads the data from the original volume.
If desired, you can perform either a background (non-blocking) or foreground
(blocking) synchronization of the snapshot volume. This is useful if you intend
to move the snapshot volume into a separate disk group for off-host processing,
or you want to turn the snapshot volume into an independent volume.
The vxsnap refresh command allows you to update the data in a snapshot, for
example, before taking a backup.
The command vxsnap reattach attaches snapshot plexes to the original volume,
and resynchronizes the data in these plexes from the original volume.
Alternatively, you can use the vxsnap restore command to restore the contents
of the original volume from a snapshot that you took at an earlier point in time.
You can also choose whether or not to keep the snapshot volume after restoration
of the original volume is complete.
See “Restoring the original volume from a snapshot” on page 363.
By default, the FastResync feature of VxVM is used to minimize the time and I/O
needed to resynchronize the data in the snapshot mirror. If FastResync is not
enabled, a full resynchronization of the data is required.
See “FastResync” on page 65.
See “Creating and managing full-sized instant snapshots” on page 373.
An empty volume must be prepared for use by full-sized instant snapshots and
linked break-off snapshots.
See “Creating a volume for use as a full-sized instant or linked break-off snapshot”
on page 370.
355Administering volume snapshots
Full-sized instant snapshots