Using Multipathing for Efficient Storage

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 simple 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

Alternatively, 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.

Configuring Multipathing

Install the device-mapper-multipath package.
```
sudo dnf install device-mapper-multipath
```
Activate the basic configuration settings of the multipathing feature.
```
sudo mpathconf --enable --with_multipathd y
```
This command also creates the /etc/multipath.conf file.
(Optional) To know the status of multipathing, type:
```
sudo mpathconf
```
Edit /etc/multipath.conf as required.

For details, see Working With the Multipathing Configuration File.

To display the current multipath configuration, run multipath -ll:

sudo multipath -ll

The command displays output similar to the following, when multipath is configured properly:

mpath1(360000970000292602744533030303730) dm-0 SUN,(StorEdge 3510|T4
size=20G features=‘0’ hwhandler=‘0’ wp=rw
|-+- policy=‘round-robin 0’ prio=1 status=active
| ‘- 5:0:0:2 sdb 8:16    active ready running
‘-+- policy=‘round-robin 0’ prio=1 status=active
  ‘- 5:0:0:3 sdc 8:32    active ready running

The sample output shows that /dev/mapper/mpath1 subsumes two paths (/dev/sdb and /dev/sdc) to 20 GB of storage in an active/active configuration using round-robin I/O path selection. The WWID that identifies the storage is 360000970000292602744533030303730 and the name of the multipath device under sysfs is dm-0.

For more information, see the mpathconf(8), multipath(8), multipathd(8), multipath.conf(5), and scsi_id(8) manual pages.

Working With the Multipathing Configuration File

Through the /etc/multipath.conf file, you can add a combination of definitions that customizes multipathing according to your system environment setup. You can obtain a commented example configuration from /usr/share/doc/device-mapper-multipath/multipath.conf.

The /etc/multipath.conf file is divided into the following typical sections:

defaults

Defines default multipath settings, which can be overridden by settings in the devices section. In turn, definitions in the devices section can be overridden by settings in the multipaths section.

blacklist

Defines devices that are excluded from multipath topology discovery. Excluded devices cannot be subsumed by a multipath device.

The example shows different ways that you can use to exclude devices: by WWID (wwid) and by device name (devnode).

blacklist_exceptions

Defines devices that are included in multipath topology discovery, even if the devices are implicitly or explicitly listed in the blacklist section.

multipaths

Defines settings for a multipath device that's identified by its WWID.

The alias attribute specifies the name of the multipath device as it will appear in /dev/mapper instead of a name based on either the WWID or the multipath group number.

devices

Defines settings for individual types of storage controller. Each controller type is identified by the vendor, product, and optional revision settings, which must match the information in sysfs for the device.

To add a storage device that DM-Multipath doesn't list as being supported, obtain the vendor, product, and revision information from the vendor, model, and rev files under /sys/block/device_name/device.

The following entries in /etc/multipath.conf would be appropriate for setting up active/passive multipathing to an iSCSI LUN with the specified WWID.

defaults {
    user_friendly_names    yes
    uid_attribute          ID_SERIAL
}

multipaths {
    multipath {
        wwid 360000970000292602744533030303730
    }
}

In this standby failover configuration, I/O continues through a remaining active network interface if a network interface fails on the iSCSI initiator.

Note:

If you edit /etc/multipath.conf, restart the multipathd service to make it re-read the file:

sudo systemctl restart multipathd

For more information about configuring entries in /etc/multipath.conf, refer to the multipath.conf(5) manual page.