The multipath-tools project is an open source project that implements I/O multipathing at the operating system level.
The project delivers an architecture and vendor-independent multipathing
solution that is based on kernel components and the following user-space
tools:
- The kernel device-mapper module (dm_multipath)
- The hotplug kernel subsystem
- The device-naming tool udev
- The user-space configuration tool multipath
- The user-space daemon multipathd
- The user-space configuration tool kpartx to create device maps from partition tables
Redundant paths that are defined in Linux appear as separate SCSI devices, one for each logical path (see Figure 1). The device-mapper provides a single block device for each logical unit (LU) and reroutes I/O over the available paths. You can partition the device-mapper multipath I/O (MPIO) devices or use them as physical volumes for LVM or software RAID.
You can use user-space components to set up the MPIO devices and
automated path retesting as follows:
- Use the multipath command to detect multiple paths to devices. It configures, lists, and removes MPIO devices.
- Use the multipathd daemon to monitor paths. The daemon tests MPIO devices for path failures and reactivates paths if they become available again.
Figure 1 shows an example multipath setup with two FCP channels for the mainframe and two HBAs for the storage subsystem.
Figure 1. Multipathing with multipath-tools
and device mapper