Veritas FlashSnap Point-In-Time Copy Solutions 5.0 AdministratorÆs Guide, HP-UX 11i v3, First Edition, May 2008

20 Point-in-time copy solutions
Implementing off-host point-in-time copy solutions
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 is usually much quicker than using alternative approaches
such as full restoration from backup media.