System information

Advanced Functions
5-25
• Creating Snapshots using GUI or CLI
After secondary volumes are chosen for the volumes need to have snapshots, you may create snapshots by the Web
GUI or CLI commands. Detailed information about GUI and CLI can be found in 2.6.6 Snapshot Volumes on page 2-23
and 4.2 Basic RAID Management on page 4-2, respectively. On Windows, you may also use the host-side CLI utility,
acs_snap.exe. After copying the executable file to the directory where you want to run the utility on your host system,
you can use the utility to create, list, and delete snapshot volumes for a LUN. However, because it communicates with the
RAID controller by the in-band interface, your primary volumes have to be exported to host computers to get commands
from the acs_snap.exe utility.
• Pausing I/O at Hosts and Applications
Before creating a snapshot, all write data on the LUN of the primary volume have to be committed to the RAID storage
and no data structure is in the inconsistent state. Otherwise, the RAID controller would capture a corrupted data image
that prohibits your operating systems or applications from using it. For example, if a money-transfer transaction
completes only reducing the source and leaves the destination intact, the snapshot taken at this moment cannot get a
balanced total sum of the money in the database. However, there are also operating systems or applications that can
successfully recover from the database with partially-done transactions by journaling algorithms.
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
(6) Mount the LUN of the primary volume at the host computers
• Deleting Snapshots
It is advised to delete any snapshot volume once you do not need it. Deleting a snapshot volume frees the space it
occupies in the secondary volume, and if there is no snapshot, the performance of the primary volume will be back to
normal. You can do it freely as long as no host is accessing it, and deleting one snapshot does not interfere with the other
snapshots of the same source 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.
Note
1. 1.Restoring data with a snapshot volume destroys the data on the primary volume.
2. If selecting a snapshot volume with LUN accessed by host computer, flush the system cache
before unmouting the LUN of the snapshot volume.