Device Mutipath参数设置

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。