相信做过Mutipath Device的童鞋对multipath.conf并不陌生。不错，这个是DM的配置文件。初次配置DM可以不修改配置文件中的任何内容，它会默认加载，当然用的参数也是默认的。有些时候我们的存储并不支持默认的配置，或者默认的配置并不适合存储的需求，这就需要对默认的参数做些修改。
下面看一下默认mutipath.conf的内容：
[root@ProsDB01 etc]# more multipath.conf
# This is a basic configuration file with some examples, for device mapper
# multipath.
# For a complete list of the default configuration values, see
# /usr/share/doc/device-mapper-multipath-0.4.7/multipath.conf.defaults
# For a list of configuration options with descriptions, see
# /usr/share/doc/device-mapper-multipath-0.4.7/multipath.conf.annotated
# Blacklist all devices by default. Remove this to enable multipathing
# on the default devices.
blacklist {
devnode "*"
}
## By default, devices with vendor = "IBM" and product = "S/390.*" are
## blacklisted. To enable mulitpathing on these devies, uncomment the
## following lines.
#blacklist_exceptions {
# device {
# vendor "IBM"
# product "S/390.*"
# }
#}
## Use user friendly names, instead of using WWIDs as names.
defaults {
user_friendly_names yes
}
##
## Here is an example of how to configure some standard options.
##
#
#defaults {
# udev_dir /dev
# polling_interval 10
# selector "round-robin 0"
# path_grouping_policy multibus
# getuid_callout "/sbin/scsi_id -g -u -s /block/%n"
# prio_callout /bin/true
# path_checker readsector0
# rr_min_io 100
# max_fds 8192
# rr_weight priorities
# failback immediate
# no_path_retry fail
# user_friendly_names yes
#}
##
## The wwid line in the following blacklist section is shown as an example
## of how to blacklist devices by wwid. The 2 devnode lines are the
## compiled in default blacklist. If you want to blacklist entire types
## of devices, such as all scsi devices, you should use a devnode line.
## However, if you want to blacklist specific devices, you should use
## a wwid line. Since there is no guarantee that a specific device will
## not change names on reboot (from /dev/sda to /dev/sdb for example)
## devnode lines are not recommended for blacklisting specific devices.
##
#blacklist {
# wwid 26353900f02796769
# devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
# devnode "^hd[a-z]"
#}
#multipaths {
# multipath {
# wwid 3600508b4000156d700012000000b0000
# alias yellow
# path_grouping_policy multibus
# path_checker readsector0
# path_selector "round-robin 0"
# failback manual
# rr_weight priorities
# no_path_retry 5
# }
# multipath {
# wwid 1DEC_____321816758474
# alias red
# }
#}
#devices {
# device {
# vendor "COMPAQ "
# product "HSV110 (C)COMPAQ"
# path_grouping_policy multibus
# getuid_callout "/sbin/scsi_id -g -u -s /block/%n"
# path_checker readsector0
# path_selector "round-robin 0"
# hardware_handler "0"
# failback 15
# rr_weight priorities
# no_path_retry queue
# }
# device {
# vendor "COMPAQ "
# product "MSA1000 "
# path_grouping_policy multibus
# }
#}
其中包含了几项参数：blacklist、defaults、multipaths、devices。这些参数对多路径的初始化信息起着决定性的作用。
这里简单介绍下这个参数的作用，详细内容可参见官方文档：http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5/html-single/DM_Multipath/#mpath_devices。
1、blacklist
这个参数用作排除磁盘的。有些磁盘比如本地盘已经做了Raid，这里就需要将本地盘和光驱设备排除，不配置多路径。语法格式配置文件中也有，一般情况sda是作为本地磁盘：
blacklist {
devnode "^sda"
}
当然，官方的建议还是采用WWID排除的方式，因为有些时候本地盘不只一个，名字也容易发生变化。
blacklist {
wwid 26353900f02796769
# devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
# devnode "^hd[a-z]"
}
WWID号可以通过multipath -ll查看，或者
/sbin/scsi_id -g -s /block/sdb
2、defaults
这个参数是个全局参数，如果设置了，针对所有的盘是生效的。
3、multipaths
这个参数是针对一个盘的，配置一个盘得相关属性
4、devices
这个参数是针对设备的，不同存储对应不同的vendor。有些时候存储上的盘并不能被DM识别到，需要在devices参数中加入存储相关的参数。优先级要高于multipaths，比如path_grouping_policy在multipath设置为multibus，而在devices中设置为failover，那么生效的是failover。
这些了解了再看一下具体的参数，虽然很多，需要改动的很少。下面是比较重要的2个参数：
path_grouping_policy
这个参数常用的两个：multipath failover，前者做负载均衡，后者是主备方式
path_checker
这个参数要根据存储类型来定。常见的几种：readsector0, tur, emc_clariion, hp_sw, directio。
http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5/html-single/DM_Multipath/#mpath_devices