multipathd

multipathd is the daemon component of the Linux multipath I/O subsystem. It is part of the multipath-tools package and is responsible for managing and monitoring multi-path storage connections in environments such as storage area networks (SANs). The daemon creates and maintains device-mapper based multipath devices (for example, /dev/mapper/mpath0) that present a single logical path to storage backed by multiple physical paths.

Multipath I/O uses multiple transport paths to a single device to improve availability and performance. multipathd discovers eligible devices, groups paths into multipath devices, and applies configuration that determines how I/O is distributed across paths, how failures are detected, and how paths are restored. It interacts with the kernel’s device-mapper layer and coordinates with the user-space multipath utilities. Features commonly supported include path failover and failback, path monitoring, and support for various path selection policies such as round-robin and service-time. Many SAN configurations also involve ALUA (asymmetric logical unit access) awareness, which multipathd can utilize to optimize path choice.

Configuration and operation are driven by the multipath.conf file, typically located at /etc/multipath.conf, which defines devices, path groups, path selectors, and various policy options. The daemon runs in the background and maintains state based on kernel events and udev, updating the presented multipath devices as paths change state. It exposes a control interface via a Unix socket, enabling administrators to query status, add or remove paths, or trigger reconfiguration without a reboot.

Usage of multipathd is common in servers connected to redundant storage arrays, especially with Fibre Channel or iSCSI, to ensure high availability and resilient I/O performance. It works in concert with the multipath command-line tools and the device-mapper subsystem to provide a unified, fault-tolerant view of persistent storage.