什么是多路径
普通的电脑主机都是一个硬盘挂接到一个总线上，这里是一对一的关系。而到了有光纤组成的SAN环境，或者由iSCSI组成的IPSAN环境，由于主机和存储通过了光纤交换机或者多块网卡及IP来连接，这样的话，就构成了多对多的关系。也就是说，主机到存储可以有多条路径可以选择。主机到存储之间的IO由多条路径可以选择。每个主机到所对应的存储可以经过几条不同的路径，如果是同时使用的话，I/O流量如何分配？其中一条路径坏掉了，如何处理？还有在操作系统的角度来看，每条路径，操作系统会认为是一个实际存在的物理盘，但实际上只是通向同一个物理盘的不同路径而已，这样是在使用的时候，就给用户带来了困惑。多路径软件就是为了解决上面的问题应运而生的。
多路径的主要功能就是和存储设备一起配合实现如下功能：
1.故障的切换和恢复
2.IO流量的负载均衡
3.磁盘的虚拟化
由于多路径软件是需要和存储在一起配合使用的，不同的厂商基于不同的操作系统，都提供了不同的版本。并且有的厂商，软件和硬件也不是一起卖的，如果要使用多路径软件的话，可能还需要向厂商购买license才行。比如EMC公司基于linux下的多路径软件，就需要单独的购买license。好在， RedHat和Suse的2.6的内核中都自带了免费的多路径软件包，并且可以免费使用，同时也是一个比较通用的包，可以支持大多数存储厂商的设备，即使是一些不是出名的厂商，通过对配置文件进行稍作修改，也是可以支持并运行的很好的。
1、安装多路径软件包：
device-mapper-1.02.67-2.el5
device-mapper-event-1.02.67.2.el5
device-mapper-multipath-0.4.7-48.el5
[root@RKDB01 Server]# rpm -ivh device-mapper-1.02.67-2.el5.x86_64.rpm
warning: device-mapper-1.02.67-2.el5.x86_64.rpm: Header V3 DSA signature: NOKEY, key ID 37017186
Preparing... ########################################### [100%]
package device-mapper-1.02.67-2.el5.x86_64 is already installed
[root@RKDB01 Server]# rpm -ivh device-mapper-event-1.02.67-2.el5.x86_64.rpm
warning: device-mapper-event-1.02.67-2.el5.x86_64.rpm: Header V3 DSA signature: NOKEY, key ID 37017186
Preparing... ########################################### [100%]
package device-mapper-event-1.02.67-2.el5.x86_64 is already installed
[root@RKDB01 Server]# rpm -ivh device-mapper-multipath-0.4.7-48.el5.x86_64.rpm
warning: device-mapper-multipath-0.4.7-48.el5.x86_64.rpm: Header V3 DSA signature: NOKEY, key ID 37017186
Preparing... ########################################### [100%]
package device-mapper-multipath-0.4.7-48.el5.x86_64 is already installed
2、设置开机启动，并检查安装包是否正常：
chkconfig --level 345 multipathd on
lsmod |grep
dm_multipath
[root@RKDB01 Server]# chkconfig --level 345 multipathd on
[root@RKDB01 Server]# lsmod |grep dm_multipath
dm_multipath 58969 0
scsi_dh 42561 1 dm_multipath
dm_mod 102417 4 dm_mirror,dm_multipath,dm_raid45,dm_log
[root@RKDB01 Server]#
3、配置multipathd 使其正常工作，编辑/etc/multipath.conf，开放如下内容：
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 none
path_checker readsector0
rr_min_io 100
max_fds 8192
rr_weight priorities
failback immediate
no_path_retry fail
user_friendly_names yes
}
blacklist {
wwid 26353900f02796769
devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
devnode "^hd[a-z]"
}
4、并关闭如下内容
#blacklist {
# devnode "*"
#}
#defaults {
27 # user_friendly_names yes
28 #}
5、完成之后执行如下命令发现多路径：
[root@RKDB01 Server]# modprobe dm-multipath
[root@RKDB01 Server]# multipath -F
[root@RKDB01 Server]# multipath dm-multipath
[root@RKDB01 Server]# multipath dm-round-robin
[root@RKDB01 Server]# service multipathd restart
正在关闭multipathd 端口监控程序： [确定]
正在启动守护进程multipathd： [确定]
[root@RKDB01 Server]# multipath -v2
[root@RKDB01 Server]# multipath -v2
[root@RKDB01 Server]# multipath -ll
mpath1 (3600d02310000011b16a5d57c6a1bd99a) dm-0 TOYOU,NetStor_iSUM510
[size=3.3T][features=0][hwhandler=0][rw]
\_ round-robin 0 [prio=2][ena bled]
\_ 1:0:0:0 sdb 8:16 [failed][ready]
\_ 1:0:1:0 sdc 8:32 [failed][ready]
[root@RKDB01 Server]#
6、重启服务器后，我们可以看到多路径信息了：
[root@RKDB01 ~]# ll /dev/mapper/
总计 0
crw------- 1 root root 10, 60 11-05 22:35 control
brw-rw---- 1 root disk 253, 0 11-05 22:35 mpath1
brw-rw---- 1 root disk 253, 1 11-05 22:35 mpath2
[root@RKDB01 ~]# multipath -ll
mpath2 (3600d02310000011b76128b9c63138cf4) dm-1 TOYOU,NetStor_iSUM510
[size=3.2T][features=0][hwhandler=0][rw]
\_ round-robin 0 [prio=2][active]
\_ 1:0:0:1 sdc 8:32 [active][ready]
\_ 1:0:1:1 sde 8:64 [active][ready]
mpath1 (3600d02310000011b16a5d57c6a1bd99a) dm-0 TOYOU,NetStor_iSUM510
[size=20G][features=0][hwhandler=0][rw]
\_ round-robin 0 [prio=2][active]
\_ 1:0:0:0 sdb 8:16 [active][ready]
\_ 1:0:1:0 sdd 8:48 [active][ready]
7、通过fdisk 看可以生成了DM-0/DM-1两个盘，正是上面sdc/sde,sdb/sdd多路径后出来的：
[root@RKDB01 ~]# fdisk -l
Disk /dev/sda: 299.4 GB, 299439751168 bytes
255 heads, 63 sectors/track, 36404 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sda1 * 1 38 305203+ 83 Linux
/dev/sda2 39 13092 104856255 83 Linux
/dev/sda3 13093 19619 52428127+ 83 Linux
/dev/sda4 19620 36404 134825512+ 5 Extended
/dev/sda5 19620 26146 52428096 83 Linux
/dev/sda6 26147 28757 20972826 83 Linux
/dev/sda7 28758 30324 12586896 82 Linux swap / Solaris
/dev/sda8 30325 36404 48837568+ 83 Linux
Disk /dev/sdb: 21.4 GB, 21474836480 bytes
255 heads, 63 sectors/track, 2610 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/sdb doesn't contain a valid partition table
Disk /dev/sdc: 3568.4 GB, 3568429957120 bytes
255 heads, 63 sectors/track, 433836 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/sdc doesn't contain a valid partition table
Disk /dev/sdd: 21.4 GB, 21474836480 bytes
255 heads, 63 sectors/track, 2610 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/sdd doesn't contain a valid partition table
Disk /dev/sde: 3568.4 GB, 3568429957120 bytes
255 heads, 63 sectors/track, 433836 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/sde doesn't contain a valid partition table
Disk /dev/dm-0: 21.4 GB, 21474836480 bytes
255 heads, 63 sectors/track, 2610 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/dm-0 doesn't contain a valid partition table
Disk /dev/dm-1: 3568.4 GB, 3568429957120 bytes
255 heads, 63 sectors/track, 433836 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/dm-1 doesn't contain a valid partition table
Disk /dev/sdf: 4009 MB, 4009754624 bytes
255 heads, 63 sectors/track, 487 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sdf4 * 1 488 3915744+ b W95 FAT32
Partition 4 has different physical/logical endings:
phys=(486, 254, 63) logical=(487, 125, 22)
[root@RKDB01 ~]#
8、我们同时也可以在/dev/mapper目录中查看到多路径映射的信息：
[root@RKDB01 ~]# ll /dev/mapper/
总计 0
crw------- 1 root root 10, 60 11-06 00:49 control
brw-rw---- 1 root disk 253, 2 11-06 00:49 data-data001
brw-rw---- 1 root disk 253, 0 11-06 00:49 mpath1
brw-rw---- 1 root disk 253, 1 11-06 00:49 mpath2
