Chapter 7 - Vss Installation; Theory Of Operation - Overland Storage SnapSAN S1000 User Manual

Chapter 7
This appendix describes how to download and install Overland Storage VSS software for
your SnapSAN S1000 storage array. The software includes the following items:
• Multipath I/O (MPIO) driver
To perform Multipath I/O operations in Fibre Channel or SAS environments, the
MPIO driver must be installed on a network computer running Windows.
• Volume Shadow Copy Service (VSS) optional software
To use VSS as an alternate way of creating snapshots in an iSCSI environment, install
the Overland Storage VSS software.

Theory of Operation

When setting up a system that makes use of the Snapshot feature, you must create two
separate arrays. One of these arrays holds the logical drive that contains the user's data and
is available as a standard volume within Windows. The second array is used to hold the
snapshot data. Snapshot data is the data copied from the user volume at a specific point in
the past.
When enabling the Snapshot feature on your SnapSAN S1000 logical drive holding the users
data, you must choose the area of the second array. The second array holds the snapshot
data and will be made into an internally available logical drive. A special area is created on
that second array called the Overwrite Data Area (ODA). This area is not available for
normal use by any hosts connected to the system – it is reserved purely for snapshot
purposes.
When a snapshot is initiated using a Windows requester such as the Backup Utility, the
following happens:
• I/O is temporarily frozen and cached data is written to the originating volume.
• VSS asks any registered providers if they support all of the parts of the selected
volume. If a VSS provider responds in a positive way, then VSS commands the
provider to start a snapshot.
• A command is then sent to the enclosure to tell it to perform the snapshot. All
subsequent writes to the originating volume cause the data that is about to be
overwritten to be copied to the snapshot area.
• After the snapshot has completed successfully, VSS waits for the Plug and Play device
manager to recognize that a new device has arrived. This new device is the snapshot,
itself. The snapshot is hidden, but you can expose and use it as a standard read-only
volume at some future time. This allows you to see the volume as it was at the time of
the snapshot and map it as a physical volume within the operating system.
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©2010-13 Overland Storage, Inc.
VSS Installation
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