Novell LINUX ENTERPRISE SERVER 10 SP3 - ARCHITECTURE-SPECIFIC, Руководство администратора, Страница: 50 / 132

50 SLES 10 SP3: Storage Administration Guide
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The user-space component of DM-MP takes care of automatic path discovery and grouping, as well
as automated path retesting, 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.
DM-MP protects against failures in the paths to the device, and not failures in the device itself. If
one of the active paths is lost (for example, a network adapter breaks or a fiber-optic cable is
removed), I/O is redirected to the remaining paths. If the configuration is active/passive, then the
path fails over to one of the passive paths. If you are using the round-robin load-balancing
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.
Table 5-1
Multipath I/O Features of Storage Arrays
Features of Storage Arrays Description
Active/passive controllers 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.
Active/active controllers 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.
Load balancing The Device Mapper Multipath 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 Multipath driver automatically activates the paths
between the host and the standby, making them the primary paths.