Device Multipathing Sample Setup

You would typically configure multipathing on a system that can access storage on a Fibre Channel-based storage area network (SAN), or on an iSCSI initiator if redundant network connections exist between the initiator and the target.

Figure 9-1 shows a simple DM-Multipath configuration where two I/O paths are configured between a server and a disk on a SAN-attached storage array:

  • Between host bus adapter hba1 on the server and controller ctrl1 on the storage array.

  • Between host bus adapter hba2 on the server and controller ctrl2 on the storage array.

Figure 9-1 DM-Multipath Mapping of Two Paths to a Disk over a SAN


The diagram shows a DM-Multipath configuration where two I/O paths are configured between a server and a disk on a SAN-attached storage array. DM-Multipath creates a single multipath device, /dev/mapper/mpathN, that subsumes the underlying devices, /dev/sdc and /dev/sdf.

Without DM-Multipath, the system treats each path as being separate even though both paths connect to the same storage device. DM-Multipath creates a single multipath device, /dev/mapper/mpathN , that subsumes the underlying devices, /dev/sdc and /dev/sdf.

The multipathing service (multipathd) handles I/O from and to a multipathed device in one of the following ways:

Active/Active

I/O is distributed across all available paths, either by round-robin assignment or dynamic load-balancing.

Active/Passive (standby failover)

I/O uses only one path. If the active path fails, DM-Multipath switches I/O to a standby path. This is the default configuration.

Note:

DM-Multipath can provide failover in the case of path failure, such as in a SAN fabric. Disk media failure must be handled by using either a software or hardware RAID solution.

The naming of multipath devices is managed by multipathing's user_friendly_names property in the multipath.conf file. If set to no, then the devices are named based on their World Wide Identifiers (WWIDs) in /dev/mapper/WWID . WWIDs are unique to their respective devices.

If the property is set to yes, the devices are mapped as /dev/mapper/mpathN , where N is the multipath group number. In addition, you can use the alias attribute to assign meaningful names to the devices. See Working With the Multipathing Configuration File.

To check the status of user_friendly_names and other DM-multipath settings, issue the mpathconf command, for example:

sudo mpathconf

Information similar to the following is displayed:

multipath is enabled
find_multipaths is enabled
user_friendly_names is enabled
dm_multipath modules is loaded
multipathd is running

Or, you can view the settings in /etc/multipath.conf.

You can use the multipath device in /dev/mapper to reference the storage in the same way as you would any other physical storage device. For example, you can configure it as an LVM physical volume, file system, swap partition, Automatic Storage Management (ASM) disk, or raw device.