15.1 Understanding Multipath I/O

Multipath I/O software resolves multiple paths to a device into a single device and manages the traffic flow across the paths transparently for file systems on the devices. NSS on Linux does not provide an LVM-based software solution for managing multiple paths like the Media Manager multipath solution on NetWare. Instead, you can use Linux multipath I/O tools to configure and manage multiple paths for devices where you want to create NSS software RAIDs, pools, and volumes. You can also use solutions from the storage array vendor or third-party vendor.

Devices have multiple connection paths when you implement hardware configurations such as the following:

  • The server has multiple host bus adapters for connection to external devices.

  • The external storage device has multiple interconnects for connection to one or more host bus adapters.

  • A server with multiple host bus adapters is connected to a storage device through intermediate devices, such as a Fibre Channel SAN switch.

In a Linux host, when there are multiple paths to a storage controller, each path appears as a separate block device, which results in multiple block devices for single LUN. The Device Mapper Multipath utility detects multiple paths with the same LUN WWID, and creates a new multipath device with that WWID.

For example, a host with two HBAs attached to a storage controller with two ports via a single unzoned Fibre Channel switch sees four block devices:

  • /dev/sdb
  • /dev/sdc
  • /dev/sdd
  • /dev/sde

Device Mapper Multipath creates a single block device, /dev/mpath/mpath1 that reroutes I/O through those four underlying block devices.