Accusys ExaRAID GUI - Manual (Page 207)

Advanced Functions
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• Reducing Data-freezing Time for Backup or Replication
When doing backup or data replication, the data of a volume has to be
frozen to maintain the data consistency by pausing the I/O access of the
applications. With the snapshot function, a copy of a volume can be
created instantaneously, and the backup or replication operations can
be performed on the snapshot volume, so the time to freeze a volume for
backup can be largely reduced.
• Testing Applications with Real-World Data
Because snapshot is created from production volume and its data is
writeable independently from the original volume, you can use the
snapshot to test new applications to find potential problems after
software upgrade or patch.
• Supporting SAN-based Applications
A snapshot volume can be exported to other host computers to offload
the backup or other applications from the host computers owning the
working volume. This improves the system performance as well as frees
you from installing all applications on all the host computers.
5.4.2 How Snapshot Works
• Snapshot Volume Pair and Copy-On-Write Operations
Before creating snapshots for a working volume, another volume
(secondary volume) is needed to be associated with the working volume
(primary volume) to form a snapshot volume pair. You can use JBOD
disks, logical disks, or volumes as primary or secondary volumes. After a
snapshot is created, the write commands to the primary volume will
invoke “copy-on- write” (COW) operation, which copies the old data
from the primary volume to the secondary volume before updating the
primary volume with the new data. The COW operation preserves the
data, and the primary volume can still be accessed.
• Secondary Volume and Lookup Table
A snapshot volume is a virtualized entity, which leverages the data and
space both on the primary and secondary volume. When an I/O
command reads the snapshot volume, it retrieves the data either from
the primary volume if the data is not updated or from the secondary
volume if the data has been changed. And writes to a snapshot volume
will be also stored in the secondary volume. A lookup table is maintained
Note
Please test your backup scenarios and restored data with your
applications. For sophisticated applications, like database, the
restored data of the primary volume has to be in sync with data on
other volumes in order to ensure proper operations of the
applications.