第 2 章 多路径设备
Without DM Multipath, each path from a server node to a storage controller is treated by the system as a separate device, even when the I/O path connects the same server node to the same storage controller. DM Multipath provides a way of organizing the I/O paths logically, by creating a single multipath device on top of the underlying devices.
2.1. 多路径设备识别器
每个多路径设备都有一个全球识别号（WWID），它是一个全球唯一的无法更改的号码。默认情况下，多路径设备的名称将设为它的 WWID。另外，您还可以在多路径配置文件中设置
user_friendly_names 选项，该选项可将别名设为格式为 mpathn 的节点唯一名称。
For example, a node with two HBAs attached to a storage controller with two ports by means of a single unzoned FC switch sees four devices:
/dev/sda, /dev/sdb, dev/sdc, and /dev/sdd. DM Multipath creates a single device with a unique WWID that reroutes I/O to those four underlying devices according to the multipath configuration. When the user_friendly_names configuration option is set to yes, the name of the multipath device is set to mpathn.
When new devices are brought under the control of DM Multipath, the new devices may be seen in two different places under the
/dev directory: /dev/mapper/mpathn and /dev/dm-n.
/dev/mapper中的设备是在引导过程中生成的。可使用这些设备访问多路径设备，例如在生成逻辑卷时。- Any devices of the form
/dev/dm-nare for internal use only should never be used by the administrator directly.
For information on the multipath configuration defaults, including the
user_friendly_names configuration option, see 第 4.3 节 “配置文件默认设置”.
You can also set the name of a multipath device to a name of your choosing by using the
alias option in the multipaths section of the multipath configuration file. For information on the multipaths section of the multipath configuration file, see 第 4.4 节 “多路径设备配置属性”.
