HP B6960-96035, Руководство по концепциям

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VSS filesystem backup and restore
Some applications are not aware of the Volume Shadow Copy service. Such
applications cannot guarantee consistency of data during the creation of a shadow
copy. The VSS mechanism cannot coordinate the activities of these applications in
order to perform a consistent backup.
However, you can still benefit from the VSS functionality. The cooperation between
the backup application and a shadow copy provider can be still used to assure a
higher level of data consistency. Microsoft calls this state of data consistency “crash
consistent data”. This means that the VSS mechanism commits all pending I/O
operations and holds incoming writing requests during the preparation of a shadow
copy volume. In this way, all files on the filesystem are closed and unlocked when
the shadow copy is being created.
Microsoft Volume Shadow Copy functionality allows the creation of a volume shadow
copy without the participation of the applications being backed up. In this case, the
shadow copy volume is created and then backed up by Data Protector. This approach
can be used with applications that are not aware of the VSS mechanism.
IMPORTANT:
When applications that are not aware of the VSS mechanism are being backed up, data
consistency from the applications’ point of view cannot be guaranteed. Data consistency
is the same as in the event of a power failure. Data Protector cannot guarantee any data
consistency when applications are not actively participating in the creation of a shadow
copy.
The consistency of data in a VSS filesystem backup is improved in comparison to a
non-VSS filesystem backup. VSS allows you to create shadow copy backups of
volumes and exact point-in-time copies of files, including all open files. For example,
databases that are held open exclusively and files that are open due to operator or
system activity are backed up during a VSS filesystem backup. In this way, files that
have changed during the backup procedure are copied correctly.
The advantages of VSS filesystem backup are as follows:
• A computer can be backed up while applications and services are running.
Therefore, applications can continue to write data to the volume during a backup.
• Files that are open are no longer skipped during the backup process because
they appear closed on the shadow copy volume at the time of the creation of the
shadow copy.
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