User Manual

Advanced Functions
5-39
In contrary to stopping the applications manually, you may use the utility
offered by your applications to force the applications to enter
“quiescent” state, in which there is no ongoing transaction and all
completed transactions have been made effective permanently. In
some systems, you may try to un-mount the LUN to force the operating
systems to flush cached data and to avoid I/O access when the snapshot
is taken.
• Dealing with Identical Data Images at Hosts
Some operating systems and applications could get confused when
seeing more than one identical volume at the same time. In Windows, if a
volume is configured as a dynamic disk, its snapshot volume will get the
same Disk ID, and the Windows Logical Disk Manager will malfunction
when both volumes are present. To avoid this problem, it is advised to
export the snapshot volumes to other host computers, like a backup
server. However, the type of operating systems to access the snapshot
volumes should be capable of recognizing the data created by the host
computer of the source volumes.
• Retaining Permanent Data of Snapshots
Because a snapshot volume serves I/O by accessing the corresponding
primary volume and secondary volume, its reliability and performance
also depends on the configurations of these two volumes. The snapshot
volume will crash if either of the two volumes is damaged. To completely
retain the data in the snapshot volume for data protection or to avoid
performance degradation, it is advised to copy the data from a snapshot
volume to another volume or another RAID system. Applications that
need to keep the snapshot data for a long time, like doing data mining,
compression, or archival, having an independent data image is more
suitable.
• Restoring Data with a Snapshot
To restore data of a primary volume from a selected snapshot volume,
please follow the steps below:
(1) Unmount the LUN of the primary volume at the host computers
(2) Remove the LUN mappings of the primary volume
(3) Remove the LUN mappings of the snapshot volume (optional)
(4) Issue the snapshot restore command from GUI or CLI
(5) Restore the LUN mappings for the primary volume
Note
A snapshot volume would crash when any of its primary volume,
secondary volume, or spare COW volume crashes. A primary volume
would crash if either secondary volume or spare COW volume
crashes while it is in the restoring state. In the cases above, please
delete the volume pair.