ServerNet Nomadic Disk User's Guide

ServerNet Nomadic Disk Overview
ServerNet Nomadic Disk User’s Guide—425485-001
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Remote Disk Drive Mirroring
The ServerNet Nomadic Disk subsystem is a combination of hardware and software that
allows you to mirror 45xx disk modules at remote nodes and switch the ownership of
the disk drives in those modules from one node to another. Remote mirroring and
fiber-optic switching are central to understanding ServerNet Nomadic Disk technology:
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The primary and mirror disk drives of one database can be at different physical
locations as described in Remote Disk Drive Mirroring on page 1-2.
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In configurations with switch modules, you can switch the path of fiber-optic
signals between an external disk module and two nodes as described in Physically
Switching the Disk Drives on page 1-2.
In addition to physical switching, changing the fiber-optic signal path also requires
some logical switching. Logical switching, as described in Logically Switching a
Database on page 1-5, corrects naming inconsistencies that arise when a database is
moved to a node with a different name.
Not all applications need both remote mirroring and fiber-optic switching; each can be
used independently. However, combining both provides the security of geographic
separation and the convenience of not requiring cables to be rerouted.
Remote Disk Drive Mirroring
When disk drives are mirrored, a pair of identical disk drives is used as a single logical
volume. One disk drive is the primary disk drive, and the other is the mirror disk drive.
Each byte of data written to the primary disk drive is also written to the mirror disk
drive. If the primary disk drive fails, the mirror disk drive can be used to continue
operations.
Remote mirroring increases the distance between the primary and mirror disk drives.
Mirror disk drives can be placed in another room, building, or city. Installation of
mirrored disk drives in geographically remote locations can protect a database on those
volumes from local environmental hazards, such as fires, floods, and earthquakes.
Because ServerNet Nomadic Disk technology relies on traditional disk mirroring, the
database on both disk drives of a mirrored volume is always synchronized and,
therefore, up to date.
Physically Switching the Disk Drives
For an external disk drive connected to separate nodes, fiber-optic switching changes the
path of fiber-optic signals between an external disk module and one of the nodes to
between the external disk drive and the other node. This switchover feature eliminates
the need to disconnect the cables between a node and an external disk drive and then
reconnecting the cable to another node or external disk drive. Because the disk drives
can be switched between nodes relatively quickly, the ServerNet Nomadic Disk
subsystem minimizes the length of application outages during planned system outages
and provides faster recovery from unplanned system outages.