Using Multipathing for Efficient Storage

6 Using Multipathing for Efficient Storage

Multiple paths to storage devices provide connection redundancy, failover capability, load balancing, and improved performance. Device-Mapper Multipath (DM-Multipath) is a multipathing tool that enables you represent multiple I/O paths between a server and a storage device as a single path.

Device Multipathing Sample Setup

You would be most likely to configure multipathing with a system that can access storage on a Fibre Channel-based storage area network (SAN). You can also use multipathing on an iSCSI initiator if redundant network connections exist between the initiator and the target.

Figure 6-1 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 6-1 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 it connects the server 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.

You can configure the multipathing service (multipathd) to handle 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.

Configuring Multipathing

The following procedure describes how to set up a simple multipath configuration on a server with access to SAN-attached storage.

Install the device-mapper-multipath package.
```
sudo yum install device-mapper-multipath
```
Initiate 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 determine the status of multipathing, use the following command:
```
sudo mpathconf
```
Edit /etc/multipath.conf, as required.

For details, see Working With the Multipathing Configuration File.

If you want to display the current multipath configuration, use the multipath -ll command, for example:
```
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 previous 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 the 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

By using 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, and which in turn can be overridden by settings in the multipaths section.

blacklist

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

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

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 is 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.

To obtain the WWID of a SCSI device, use the scsi_id command:

sudo scsi_id --whitelisted --replace-whitespace --device=device_name

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.

You can find details of the storage arrays that DM-Multipath supports and their default configuration values in /usr/share/doc/device-mapper-multipath-version/multipath.conf.defaults, which you can use as the basis for entries in the /etc/multipath.conf file.

To add a storage device that DM-Multipath does not 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 the /etc/multipath.conf file 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 interfaces fails on the iSCSI initiator.

Note:

If you edit the /etc/multipath.conf file, restart the multipathd service to make so that the file is re-read.

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