Novell LINUX ENTERPRISE SERVER 10 SP2 - STORAGE ADMINISTRATION GUIDE 05-15-2009 Administration Manual page 50

configuration, the traffic is balanced across the remaining healthy paths. If all active paths fail,
inactive secondary paths must be waked up, so failover occurs with a delay of approximately 30
seconds.
If a disk array has more than one storage processor, make sure that the SAN switch has a connection
to the storage processor that owns the LUNs you want to access. On most disk arrays, all LUNs
belong to both storage processors, so both connections are active.
NOTE: On some disk arrays, the storage array manages the traffic through storage processors so
that it presents only one storage processor at a time. One processor is active and the other one is
passive until there is a failure. If you are connected to the wrong storage processor (the one with the
passive path) you might not see the expected LUNs, or you might see the LUNs but get errors when
trying to access them.
Multipath I/O Features of Storage Arrays
Table 5-1
Features of Storage Arrays
Active/passive controllers
Active/active controllers
Load balancing
Controller failover
Boot/Root device support
Device Mapper Multipath detects every path for a multipathed device as a separate SCSI device.
The SCSI device names take the form
beginning with a and issued sequentially as the devices are created, such as
and so on. If the number of devices exceeds 26, the letters are duplicated so that the next device after
/dev/sdz
50 SLES 10 SP2: Storage Administration Guide
Description
One controller is active and serves all LUNs. The second controller acts as
a standby. The second controller also presents the LUNs to the multipath
component so that the operating system knows about redundant paths. If
the primary controller fails, the second controller takes over, and it serves
all LUNs.
In some arrays, the LUNs can be assigned to different controllers. A given
LUN is assigned to one controller to be its active controller. One controller
does the disk I/O for any given LUN at a time, and the second controller is
the standby for that LUN. The second controller also presents the paths,
but disk I/O is not possible. Servers that use that LUN are connected to
the LUN's assigned controller. If the primary controller for a set of LUNs
fails, the second controller takes over, and it serves all LUNs.
Both controllers share the load for all LUNs, and can process disk I/O for
any given LUN. If one controller fails, the second controller automatically
handles all traffic.
The Device Mapper Multipath driver automatically load balances traffic
across all active paths.
When the active controller fails over to the passive, or standby, controller,
the Device Mapper Multipath driver automatically activates the paths
between the host and the standby, making them the primary paths.
Multipathing is supported for the root (
Server 10 and later. The host server must be connected to the currently
active controller and storage processor for the boot device. The
partition must be on a separate, non-multipathed partition. Otherwise, no
boot loader is written. For information, see
Multipath I/O for the Root Device," on page
will be named
/dev/sdaa
, where is an autogenerated letter for the device,
/dev/sdN N
, , and so on.
/dev/sdab
) device in SUSE Linux Enterprise
/
/boot
Section 5.8, "Configuring
67.
,
/dev/sda /dev/sdb
,