SUSE® Linux Enterprise Server 10

Chapter 5. Managing Multipath I/O for Devices / 5.2. Multipath Management Tools
	Chapter 5. Managing Multipath I/O for Devices		5.3. Supported Storage Subsystems

5.2. Multipath Management Tools

The multipath I/O support in SUSE Linux Enterprise Server is based on the Device Mapper Multipath I/O module of the Linux kernel and the multipath-tools userspace package. Use mdadm to view the status of multipathed devices.

5.2.1. Device Mapper Multipath I/O Module

The Device Mapper Multipath I/O (DM-MPIO) module provides the multipathing capability for Linux. Multipath protects against failures in the paths to the device, and not failures in the device itself. If one of the paths is lost (for example, a network adapter breaks or a fiber-optic cable is removed), I/O will be redirected to the remaining paths. If an active path fails, the DM continues to balance traffic across the healthy paths. If all active paths fail, inactive secondary paths must be waked up, so failover occurs with a delay of approximately 30 seconds.

Table 5.1. Multipath I/O Features

Features	Description
Active/passive	If the storage array has multiple controllers, and only one controller is active at a time, then only the paths from the host to the active storage controller are active. Connections to the second and subsequent controllers are passive.
Active/active	If the storage array has multiple controllers that are concurrently active, all connections from the host to the controllers are active and treated equally in a load-balanced setup.
Load balancing	The Device Mapper driver automatically load balances traffic across all active paths.
Controller failover	When the active controller fails over to the passive, or standby, controller, the Device Mapper driver automatically activates the paths between the host and the standby, making them the primary paths. When the failed primary controller is reactivated as primary, the Device Mapper driver also activates the previously-downed paths.
Boot/Root device support	Multipathing is supported for the root ( `/`) device in SUSE Linux Enterprise Server 10 and later.

Device Mapper detects every path for a multipathed device as a separate SCSI device. The SCSI device names take the form /dev/sd N, where N is an autogenerated letter for the device, beginning with a and issued sequentially as the devices are created, such as /dev/sda, /dev/sdb, and so on. If the number of devices exceeds 26, the letters are duplicated such that the next device after /dev/sdz will be named /dev/sdaa, /dev/sdab, and so on.

If multiple paths are not automatically detected, you can configure them manually in the /etc/multipath.conf file.

5.2.2. Multipath I/O Management Tools

The multipath-tools user-space package takes care of automatic path discovery and grouping. It automatically tests the path periodically, so that a previously failed path is automatically reinstated when it becomes healthy again. This minimizes the need for administrator attention in a production environment.

Table 5.2. Tools in the multipath-tools Package

Tool	Description
multipath	Scans the system for multipathed devices and assembles them.
multipathd	Waits for maps events, then executes multipath.
devmap-name	Provides a meaningful device name to udev for device maps (devmaps).
kpartx	Maps linear devmaps to partitions on the multipathed device, which makes it possible to create multipath monitoring for partitions on the device.

For a list of files included in this package, see the multipath-tools Package Description .

Ensure that the multipath-tools package is installed by entering the following at a terminal console prompt:

rpm -q multipath-tools

5.2.3. Using mdadm for Multipathed Devices

The default settings for mdadm.conf (and lvm.conf) do not work properly with multipathed devices. By default, both md and LVM2 scan physical devices only and ignore any symbolic links or device-mapper devices.

Scanning physical devices does not work for multipathed devices. Instead you must scan for multipathed devices in the /dev/disk/by-id directory.

If a previous MD installation exists, modify mdadm.conf to handle the devices correctly by ID instead of by device node path. For instructions, see Section 5.4.4, “Configuring mdadm.conf and lvm.conf to Scan Devices by UUID”.

To use software RAID with mdadm, the /etc/mdadm.conf must be set up correctly. See Section 5.12, “Using Multipathed Devices” for more information.