Veritas FlashSnap Point-In-Time Copy Solutions 5.0.1 Administrators Guide, HP-UX 11i v3, First Edition, November 2009

Disk Group Split/Join is used to split off snapshot volumes into a separate disk
group that is imported on the OHP host.
Note: As the snapshot volumes are to be moved into another disk group and then
imported on another host, their contents must first be synchronized with the
parent volumes. On reimporting the snapshot volumes, refreshing their contents
from the original volume is speeded by using FastResync.
Data integrity in volume snapshots
A volume snapshot represents the data that exists in a volume at a given point in
time. As such, VxVM does not have any knowledge of data that is cached by the
overlying file system, or by applications such as databases that have files open in
the file system. If the fsgen volume usage type is set on a volume that contains a
Veritas File System (VxFS), intent logging of the file system metadata ensures the
internal consistency of the file system that is backed up. For other file system
types, depending on the intent logging capabilities of the file system, there may
potentially be inconsistencies between in-memory data and the data in the
snapshot image.
For databases, a suitable mechanism must additionally be used to ensure the
integrity of tablespace data when the volume snapshot is taken. The facility to
temporarily suspend file system I/O is provided by most modern database software.
The examples provided in this document illustrate how to perform this operation.
For ordinary files in a file system, which may be open to a wide variety of different
applications, there may be no way to ensure the complete integrity of the file data
other than by shutting down the applications and temporarily unmounting the
file system. In many cases, it may only be important to ensure the integrity of file
data that is not in active use at the time that you take the snapshot.
Choices for snapshot resynchronization
When a snapshot volume is reattached to its original volume within a shared disk
group, there are two choices for resynchronizing the data in the volume:
■ Resynchronize the snapshot from the original volume—updates the snapshot
with data from the primary volume that has changed since the snapshot was
taken. The snapshot is then again ready to be taken for the purposes of backup
or decision support.
■ Resynchronize the original volume from the snapshot—updates the original
volume with data from the snapshot volume that has changed since the
snapshot was taken. This may be necessary to restore the state of a corrupted
database or file system, or to implement upgrades to production software, and
25Point-in-time copy solutions
Implementing off-host point-in-time copy solutions