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1.2. Overview of DM Multipath

DM Multipath can be used to provide:

冗余

DM Multipath can provide failover in an active/passive configuration. In an active/passive configuration, only half the paths are used at any time for I/O. If any element of an I/O path (the cable, switch, or controller) fails, DM Multipath switches to an alternate path.
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DM Multipath can be configured in active/active mode, where I/O is spread over the paths in a round-robin fashion. In some configurations, DM Multipath can detect loading on the I/O paths and dynamically rebalance the load.

图 1.1 “Active/Passive Multipath Configuration with One RAID Device” shows an active/passive configuration with two I/O paths from the server to a RAID device. There are 2 HBAs on the server, 2 SAN switches, and 2 RAID controllers.

图 1.1. Active/Passive Multipath Configuration with One RAID Device

在这个配置中，一个 I/O 路径通过 hba1、SAN1 以及控制器 1。第二个 I/O 路径通过 hba2、SAN2 以及控制器 2。在这个配置中有很多地方可能导致故障：

With DM Multipath configured, a failure at any of these points will cause DM Multipath to switch to the alternate I/O path.

图 1.2 “Active/Passive Multipath Configuration with Two RAID Devices” shows a more complex active/passive configuration with 2 HBAs on the server, 2 SAN switches, and 2 RAID devices with 2 RAID controllers each.

图 1.2. Active/Passive Multipath Configuration with Two RAID Devices

In the example shown in 图 1.2 “Active/Passive Multipath Configuration with Two RAID Devices”, there are two I/O paths to each RAID device (just as there are in the example shown in 图 1.1 “Active/Passive Multipath Configuration with One RAID Device”). With DM Multipath configured, a failure at any of the points of the I/O path to either of the RAID devices will cause DM Multipath to switch to the alternate I/O path for that device.

图 1.3 “Active/Active Multipath Configuration with One RAID Device” shows an active/active configuration with 2 HBAs on the server, 1 SAN switch, and 2 RAID controllers. There are four I/O paths from the server to a storage device:

在这个配置中，可将 I/O 分布到那四条路径中。

图 1.3. Active/Active Multipath Configuration with One RAID Device