Similarly, if you set the alias option for a device in the multipaths section of /etc/nf, the name is not automatically consistent across all nodes in the 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. Consistent multipath device names in a cluster 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.įor information on the multipath configuration defaults, including the user_friendly_names and alias configuration options, see DM-Multipath configuration. Use these devices to access the multipathed devices.Īny devices of the form /dev/dm-n are for internal use only and should never be used directly. The devices in /dev/mapper are created early in the boot process. 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. When the user_friendly_names configuration option is set to ‘yes’, the name of the multipath device is set to mpathn. Multipath creates a single device with a unique WWID that reroutes I/O to those four underlying devices according to the multipath configuration. Alternatively, you can set the user_friendly_names option in nf, which causes multipath to use a node-unique alias of the form mpathn as the name.įor example, a node with two host bus adapters (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. By default, the name of a multipath device is set to its WWID. Multipath device identifiersĮach multipath device has a World Wide Identifier (WWID), which is guaranteed to be globally unique and unchanging. Multipath provides a way of organizing the I/O paths logically, by creating a single device on top of the underlying paths. 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. Install the multipath-tools and multipath-tools-boot packages.Ĭreate an empty config file called /etc/nf.Įdit the nf file to modify default values and save the updated file.įor detailed setup instructions for multipath configuration see DM-Multipath configuration and DM-Multipath setup. The basic procedure for configuring your system with multipath is as follows: Setting up multipath is often a simple procedure, since it includes compiled-in default settings that are suitable for common multipath configurations. The kpartx is provided in its own package, but the multipath-tools package depends on it. It is necessary to use this command for DOS-based partitions with multipath. This daemon must be restarted for any changes to the /etc/nf file to take effect.Ĭreates device mapper devices for the partitions on a device. Provides for interactive changes to multipath devices. Monitors paths as paths fail and come back, it may initiate path group switches. 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. Reroutes I/O and supports failover for paths and path groups. The default configuration will probably work but will likely need adjustments based on your storage setup. It is a very good idea to consult your storage vendor’s installation guide for the recommended multipath configuration variables for your storage model. In some configurations, multipath can detect loading on the I/O paths and dynamically re-balance the load. Multipath can be configured in active/active mode, where I/O is spread over the paths in a round-robin fashion. If any element of an I/O path (the cable, switch, or controller) fails, multipath switches to an alternate path. In an active/passive configuration, only half the paths are used at any time for I/O. Multipath provides failover in an active/passive configuration. This chapter provides an introduction and a high-level overview of multipath. Multipathing aggregates the I/O paths, creating a new device that consists of those aggregated paths. These I/O paths are physical storage area network (SAN) connections that can include separate cables, switches, and controllers. Multi-node configuration with Docker-Composeĭistributed Replicated Block Device (DRBD)ĭevice mapper multipathing - introductionĭevice mapper multipathing (which we will refer to as multipath) allows you to configure multiple input/output (I/O) paths between server nodes and storage arrays into a single device.
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