multipath

Multipath I/O (MPIO) is a technology used in Linux and other operating systems to provide multiple physical paths between a server and its storage. The goal is to enhance the availability and performance of storage systems by allowing multiple paths for data to travel, improving both redundancy and load balancing.

Key Concepts

  1. Multipath Devices: A multipath device is a logical device that combines multiple physical paths (e.g., cables, switches) to a storage array. The OS and storage system can communicate with the device using any of these paths, with automatic failover to another path in case one fails.

  2. Path Grouping: Multipath devices can consist of several "paths" (physical connections), grouped into a single logical device. Path grouping determines how these paths are used for I/O operations.

  3. Failover and Load Balancing: If one path fails, another path will take over the I/O operations, ensuring high availability. On the other hand, load balancing algorithms allow distributing I/O requests across multiple available paths to optimize performance.

  4. Multipath Daemon: In Linux, the multipath daemon (multipathd) is responsible for managing multipath devices. It handles the discovery of paths, failure detection, and load balancing.

  5. Device Mapper Multipath: Linux uses the Device Mapper to create a virtual device from multiple paths, ensuring that the kernel treats it as a single device. This device is then used by applications like databases and file systems.

How Multipath Works

  1. Device Discovery: Multipath discovers devices by looking at their unique World Wide Identifier (WWID) or other characteristics, such as vendor and model. When a device has multiple physical paths, the multipath subsystem treats these as a single logical device.

  2. Path Failover: If one of the paths fails (e.g., a cable is unplugged), the multipath system will detect the failure and automatically reroute traffic to the next available path, ensuring continuous access to the device.

  3. Path Selection Algorithm: Path selection algorithms control how traffic is distributed across multiple paths. Common algorithms include:

    • Round-robin: I/O operations are alternated between paths in a circular order.

    • Least Queue Depth: Selects the path with the least number of pending I/O operations.

    • Service Time: Chooses the path with the fastest response time.

  4. Multipath Configuration: Multipath behavior can be customized by editing configuration files (such as /etc/multipath.conf) to set up things like path failover policies, device-specific settings, and blacklist options.

Configuration Files

  • /etc/multipath.conf: This is the main configuration file for the multipath daemon in Linux. It allows administrators to customize multipath device behavior, set policies for path selection, define device-specific rules, and blacklist certain devices.

Common Commands for Multipath

  • multipath: This command is used to display the current state of multipath devices.

    multipath -ll

    This shows detailed information about all multipath devices, including path status and device aliases.

  • multipath -v3: Displays verbose output, showing the internal workings of the multipath subsystem, useful for debugging.

  • multipathd: This is the daemon that manages multipath devices. It can be manually restarted to apply changes or troubleshoot.

    systemctl restart multipathd

  • multipath -f <device>: This command flushes the multipath device specified, effectively removing it from the multipath list.

Example /etc/multipath.conf

Here is an example configuration file to define how multipath should behave:

defaults {
    user_friendly_names yes
    find_multipaths yes
    path_selector "round-robin 0"
    failback immediate
    no_path_retry 5
}

blacklist {
    devnode "^sda"
}

multipaths {
    multipath {
        wwid 3600508b1001cce8d0000900000a22000
        alias mpath1
        path_grouping_policy multibus
        path_checker tur
        failback immediate
    }
}

devices {
    device {
        vendor "DELL"
        product "MD3000"
        path_grouping_policy group_by_prio
        path_checker tur
        prio_callout "/sbin/mpath_prio_alua"
        no_path_retry 5
    }
}

Key Sections in multipath.conf

  • defaults: Sets global options like path_selector for path selection algorithms and failback for controlling failover behavior.

  • multipaths: Configures specific multipath devices using their WWID, such as setting aliases and path policies.

  • blacklist: Used to blacklist certain devices, preventing them from being managed by multipath.

  • devices: Used for setting device-specific configurations based on vendor/product.

Multipath in High Availability (HA) Systems

Multipath I/O is critical in high-availability environments, such as large data centers or cloud environments, where uninterrupted access to storage is vital. By using multiple paths, multipath I/O ensures that if one path fails due to hardware issues (e.g., cable failure or switch malfunction), data traffic will automatically switch to another path, preventing downtime.

Conclusion

Multipath I/O enhances redundancy, fault tolerance, and performance in storage systems. It is widely used in enterprise environments, where continuous access to storage is necessary, and high-availability and performance optimization are key. By configuring multipath devices correctly and using path selection algorithms, administrators can ensure reliable, optimized access to storage resources.

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