Device Mapper Multipathing - Introduction

Device Mapper Multipath will be referred here as multipath only.

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 an introduction and a high-level overview of multipath.

Overview

 
Multipath can be used to provide:

  • Redundancy
     
    multipath can provide failover in an active/passive configuration. In an active/passive configuration, only half the paths are used at any time for I/O. If any element of an I/O path (the cable, switch, or controller) fails, multipath switches to an alternate path.

  • Improved Performance
     
    Multipath can be configured in active/active mode, where I/O is spread over the paths in a round-robin fashion. In some configurations, multipath can detect loading on the I/O paths and dynamically re-balance the load.

Storage Array Overview

 
It is a very good idea to consult your storage vendor installation guide for the recommended multipath configuration variables for your storage model. The default configuration will probably work but will likely need adjustments based on your storage setup.

Multipath Components

 

Component                                       Description
dm_multipath kernel module Reroutes I/O and supports failover for paths and path groups.
multipath command Lists and configures multipath devices. Normally started up with /etc/rc.sysinit, it can also be started up by a udev program whenever a block device is added or it can be run by the initramfs file system.
multipathd daemon Monitors paths; as paths fail and come back, it may initiate path group switches. Provides for interactive changes to multipath devices. This daemon must be restarted for any changes to the /etc/multipath.conf file to take effect.
kpartx command Creates device mapper devices for the partitions on a device It is necessary to use this command for DOS-based partitions with multipath. The kpartx is provided in its own package, but the multipath-tools package depends on it.

Multipath Setup Overview

 
multipath includes compiled-in default settings that are suitable for common multipath configurations. Setting up multipath is often a simple procedure. The basic procedure for configuring your system with multipath is as follows:

  1. Install the multipath-tools and multipath-tools-boot packages

  2. Create an empty config file called /etc/multipath.conf

  3. Edit the multipath.conf file to modify default values and save the updated file.

  4. Start the multipath daemon

  5. Update initial ramdisk

For detailed setup instructions for multipath configuration see DM-Multipath Configuration and DM-Multipath Setup.

Multipath Devices

 
Without multipath, each path from a server node to a storage controller is treated by the system as a separate device, even when the I/O path connects the same server node to the same storage controller. Multipath provides a way of organizing the I/O paths logically, by creating a single device on top of the underlying paths.

Multipath Device Identifiers

 
Each multipath device has a World Wide Identifier (WWID), which is guaranteed to be globally unique and unchanging. By default, the name of a multipath device is set to its WWID. Alternately, you can set the user_friendly_names option in multipath.conf, which causes multipath to use a node-unique alias of the form mpathn as the name.

For example, a node with two HBAs attached to a storage controller with two ports via a single unzoned FC switch sees four devices: /dev/sda, /dev/sdb, /dev/sdc, and /dev/sdd. Multipath creates a single device with a unique WWID that reroutes I/O to those four underlying devices according to the multipath configuration.

When the user_friendly_names configuration option is set to yes, the name of the multipath device is set to mpathn. When new devices are brought under the control of multipath, the new devices may be seen in two different places under the /dev directory: /dev/mapper/mpathn and /dev/dm-n.

  • The devices in /dev/mapper are created early in the boot process. Use these devices to access the multipathed devices.

  • Any devices of the form /dev/dm-n are for internal use only and should never be used directly.

You can also set the name of a multipath device to a name of your choosing by using the alias option in the multipaths section of the multipath configuration file.

For information on the multipath configuration defaults, including the user_friendly_names and alias configuration options, see DM-Multipath Configuration.

Consistent Multipath Device Names in a Cluster

 
When the user_friendly_names configuration option is set to yes, the name of the multipath device is unique to a node, but it is not guaranteed to be the same on all nodes using the multipath device. Similarly, if you set the alias option for a device in the multipaths section of /etc/multipath.conf, the name is not automatically consistent across all nodes in the cluster.

This should not cause any difficulties if you use LVM to create logical devices from the multipath device, but if you require that your multipath device names be consistent in every node it is recommended that you leave the user_friendly_names option set to no and that you not configure aliases for the devices.

If you configure an alias for a device that you would like to be consistent across the nodes in the cluster, you should ensure that the /etc/multipath.conf file is the same for each node in the cluster by following the same procedure:

  1. Configure the aliases for the multipath devices in the in the multipath.conf file on one machine.

  2. Disable all of your multipath devices on your other machines by running the following commands:

    # systemctl stop multipath-tools.service
    # multipath -F
    
  3. Copy the /etc/multipath.conf file from the first machine to all the other machines in the cluster.

  4. Re-enable the multipathd daemon on all the other machines in the cluster by running the following command:

    # systemctl start multipath-tools.service
    

When you add a new device you will need to repeat this process.

Multipath Device attributes

 
In addition to the user_friendly_names and alias options, a multipath device has numerous attributes. You can modify these attributes for a specific multipath device by creating an entry for that device in the multipaths section of /etc/multipath.conf.

For information on the multipaths section of the multipath configuration file, see DM-Multipath Configuration.

Multipath Devices in Logical Volumes

 
After creating multipath devices, you can use the multipath device names just as you would use a physical device name when creating an LVM physical volume.

For example, if /dev/mapper/mpatha is the name of a multipath device, the following command will mark /dev/mapper/mpatha as a physical volume:

# pvcreate /dev/mapper/mpatha

You can use the resulting LVM physical device when you create an LVM volume group just as you would use any other LVM physical device.

Note

If you attempt to create an LVM physical volume on a whole device on which you have configured partitions, the pvcreate command will fail.

When you create an LVM logical volume that uses active/passive multipath arrays as the underlying physical devices, you should include filters in the lvm.conf to exclude the disks that underlie the multipath devices. This is because if the array automatically changes the active path to the passive path when it receives I/O, multipath will failover and failback whenever LVM scans the passive path if these devices are not filtered.

For active/passive arrays that require a command to make the passive path active, LVM prints a warning message when this occurs. To filter all SCSI devices in the LVM configuration file (lvm.conf), include the following filter in the devices section of the file.

filter = [ "r/block/", "r/disk/", "r/sd.*/", "a/.*/" ]

After updating /etc/lvm.conf, it’s necessary to update the initrd so that this file will be copied there, where the filter matters the most, during boot.

Perform:

update-initramfs -u -k all

Note

Every time either /etc/lvm.conf or /etc/multipath.conf is updated, the initrd should be rebuilt to reflect these changes. This is imperative when blacklists and filters are necessary to maintain a stable storage configuration.

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