User Manual

Advanced Functions
5-38
Another consideration in estimate reserved capacity is that because the
COW operations are done by chunks, consisting of multiple consecutive
sectors, more space is required than the actual data being modified.
If the space of a secondary volume is fully occupied, the data on the
snapshot volumes will be corrupted. Be aware of applications that would
change huge amount of data on a source volume, like video recording
and file system defragmentation. If all the data will be changed, ensure
that the secondary volume’s capacity is set to 105% of the size of the
source volume.
You will also properly set the RAID attributes of the secondary volume
depending on your performance and reliability requirements for the
snapshot volumes. It is advised that the secondary volumes are located
on the different disk group of the primary volumes, such that the COW
operation and the I/O access to the snapshot volume can be done more
efficiently.
• 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 2-35 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.
Note
1. Set spare COW volumes to avoid data loss in the snapshot
volumes when the space of the secondary volume is overflow.
2. Expand secondary volume or spare COW volume to accommodate
more differential data