VERITAS FlashSnap Point-In-Time-Copy Solutions Administrator's Guide

Implementing Off-Host Point-In-Time Copy Solutions
16 VERITAS FlashSnap Point-In-Time Copy Solutions Administrator’s Guide
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.