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APPENDIX B Multipath Configuration
This appendix provides examples of client-side multipath setup for a Violin Memory
Gateway. The following topics are covered:
• Multipath Overview on page 55
• Multipath Setup on page 56
Note: The examples in this appendix are guidelines for (client-side) Red Hat
Enterprise Linux 6 DM Multipath. For more information, see to the Red Hat Web
site.
Multipath Overview
Multipath I/O is commonly used when there is more than one physical path
between the system CPU and the storage devices. The redundant paths are
leveraged to increase performance and fault-tolerance.
Device Mapper Multipath (DM Multipath, Red Hat Enterprise Linux) is the
multipathing utility used in the example presented in this appendix. DM Multipath is
a generic framework that maps each physical path to a SCSI device represented
as a separate /dev entry in a form of a block device. DM Multipath provides means
of combining multiple such block devices into a single block device. DM Multipath
uses Device Mapper kernel framework for mapping IOs between individual paths
and user-space daemon multipathd for providing configuration and policy for the
mapped device.
To determine which devices (and paths) correspond to a particular LUN, DM
Multipath initiates a query. The response to the query returns the LUN serial
number (among other information). The serial number for a LUN remains the same,
regardless of the number paths that are associated with the LUN. Using the LUN
serial number, DM Multipath can determine how many paths are associated with
that particular LUN. The mechanism for getting the serial number is controlled by
the getuid_callout parameter, described in the next section.
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