Ds3500 Fc Storage Configuration On Sles 11 Using Dmm; Dmm Overview - IBM System Storage DS3500 Introduction And Implementation Manual

Draft Document for Review March 28, 2011 12:24 pm

20.4 DS3500 FC storage configuration on SLES 11 using DMM

In this section we discuss how to attach a DS3500 logical drive to a SuSE Linux Enterprise
Server 11 using Device Mapper Multipath (DMM) module. This multipath driver is an
alternative to RDAC when its not supported by the linux kernel.
Like RDAC, Device Mapper Multipath allows you to configure multiple I/O paths between
server nodes and storage arrays creating a new virtual device that manages the different
paths in order to provide failover in an active/passive storage subsystem.

20.4.1 DMM Overview

Device mapper is a block subsystem that provides layering mechanism for block devices.
Device mapper can be written to provide a specific functionality on top of a block device.
DMM resolves all the issues that arise in accessing a multipathed device in Linux. It also
provides a consistent user interface for storage devices provided by multiple vendors. There
is only one block device (/dev/mapper/XXX) for a LUN and this is the device created by device
mapper. Paths are grouped into priority groups, and one of the priority group will be used for
I/O, and is called active. A path selector selects a path in the priority group to be used for an
I/O based on some load balancing algorithm (for example round-robin).
When a I/O fails in a path, that path gets disabled and the I/O is re-tried in a different path in
the same priority group. If all paths in a priority group fails, a different priority group which is
enabled will be selected to send I/O.
DMM consists of 4 components:
1. dm-multipath  This module is responsible for the multipathing decisions in event of a
failure.
2. multipath  This command is used to detect multiple devices for failover, to list the
available paths and to remove virtual devices.
3. multipathd daemon  This daemon constantly checks the paths to mark them as failed
when it finds a path is faulty and switches the I/O to the next enable priority group. It keeps
checking the failed path, once the failed path comes alive, based on the failback policy, it
can activate the path.
4. multipathd command  When used with -k parameter it is possible to run a command line
interface as show in
Example 20-11 multipathd command line
# multipathd -k
multipathd> help
multipath-tools v0.4.8 (08/02, 2007)
CLI commands reference:
list|show paths
list|show paths format $format
list|show status
list|show maps|multipaths
list|show maps|multipaths status
list|show maps|multipaths stats
list|show maps|multipaths topology
list|show topology
list|show map|multipath $map topology
Chapter 20. SuSE Linux configuration guide
Linux Config.fm
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