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This does not work for multipathed devices as there we have to omit all physical devices and scan
devices in /dev/disk/by-id only as these are the correct multipathed devices.
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
lvm.conf to Scan Devices by UUID," on page
5.1.5 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.
Multipath I/O Features
Table 5-1
Features
Active/passive
Active/active
Load balancing
Controller failover
Boot/Root device support
Device Mapper detects every path for a multipathed device as a separate SCSI device. The SCSI
device names take the form /dev/sdN, 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.
Section 5.2.3, "Configuring mdadm.conf and
45.
Description
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.
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.
The Device Mapper driver automatically load
balances traffic across all active paths.
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 automatically
activates the previously-downed paths, too.
Multipathing is supported for the root (/) device in
SUSE Linux Enterprise Server 10 and later.
Managing Multipath I/O for Devices
43
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