Chapter 1. Device Mapper Multipathing

1.1. New and Changed Features
1.2. Overview of DM-Multipath
1.3. Storage Array Support
1.4. DM-Multipath Components
1.5. DM-Multipath Setup Overview

Device mapper multipathing (DM-Multipath) allows you to configure multiple I/O paths between server nodes and storage arrays into a single device. These I/O paths are physical SAN connections that can include separate cables, switches, and controllers. Multipathing aggregates the I/O paths, creating a new device that consists of the aggregated paths.

This chapter provides a summary of the features of DM-Multipath that are new for the initial release of Red Hat Enterprise Linux 6. Following that, this chapter provides a high-level overview of DM Multipath and its components, as well as an overview of DM-Multipath setup.

1.1. New and Changed Features

This section lists new and changed features of DM-Multipath that are included with the initial release of Red Hat Enterprise Linux 6.

For the Red Hat Enterprise Linux 6 release, the initial DM-Multipath setup procedure for a basic failover configuration has changed. You can now create the DM-Multipath configuration file and enable DM-Multipath with the mpathconf configuration utility, which can also load the device-mapper-multipath module, start the multipathd daemon, and set chkconfig to start the daemon automatically on reboot.
For information on the new setup procedure, see Section 3.1, “Setting Up DM-Multipath”. For more information on the mpathconf command, see the mpathconf(5) man page.
The Red Hat Enterprise Linux 6 release provides a new mode for setting up multipath devices, which you set with the find_multipaths configuration file parameter. In previous releases of Red Hat Enterprise Linux, multipath always tried to create a multipath device for every path that was not explicitly blacklisted. In Red Hat Enterprise Linux 6, however, if the find_multipath configuration parameter is set to yes, then multipath will create a device only if one of three conditions are met:
- There are at least two non-blacklisted paths with the same WWID.
- The user manually forces the device creation, by specifying a device with the multipath command.
- A path has the same WWID as a multipath device that was previously created (even if that multipath device does not currently exist). For instructions on the procedure to follow if you have previously created multipath devices when the find_multipaths parameter was not set, see Section 4.2, “Configuration File Blacklist”.
This feature should allow most users to have multipath automatically choose the correct paths to make into multipath devices, without having to edit the blacklist.
For information on the find_multipaths configuration parameter, see Section 4.3, “Configuration File Defaults”.
The Red Hat Enterprise Linux 6 release provides two new path selector algorithms which determine which path to use for the next I/O operation: queue-length and service-time. The queue-length algorithm looks at the amount of outstanding I/O to the paths to determine which path to use next. The service-time algorithm looks at the amount of outstanding I/O and the relative throughput of the paths to determine which path to use next. For more information on the path selector parameters in the configuration file, see Chapter 4, The DM-Multipath Configuration File.
In the Red Hat Enterprise Linux 6 release, priority functions are no longer callout programs. Instead they are dynamic shared objects like the path checker functions. The prio_callout parameter has been replaced by the prio parameter. For descriptions of the supported prio functions, see Chapter 4, The DM-Multipath Configuration File.
In Red Hat Enterprise Linux 6, the multipath command output has changed format. For information on the multipath command output, see Section 5.5, “Multipath Command Output”.
In the Red Hat Enterprise Linux 6 release, the location of the multipath bindings file is /etc/multipath/bindings.
The Red Hat Enterprise Linux 6 release provides three new defaults parameters in the multipath.conf file: checker_timeout, fast_io_fail_tmo, and dev_loss_tmo. For information on these parameters, see Chapter 4, The DM-Multipath Configuration File.
When the user_friendly_names option in the multipath configuration file is set to yes, the name of a multipath device is of the form mpathn. For the Red Hat Enterprise Linux 6 release, n is an alphabetic character, so that the name of a multipath device might be mpatha or mpathb. In previous releases, n was an integer.