在Linux平臺一部分儲存產品使用作業系統自帶的多路徑軟體，包括最常見的HP和IBM的部分儲存產品，在Linux自帶的多路徑軟體叫做multipath，這篇文章以HP EVA系列儲存在Linux平臺的使用為例，詳細的在Linux平臺配置和使用多路徑。

那麼問題就來了，到底什麼是多路徑？

普通的電腦主機都是一個硬碟掛接到一個匯流排上，這裡是一對一的關係。而到了有光纖組成的SAN環境，或者由iSCSI組成的IPSAN環境，由於主機和儲存透過了光纖交換機或者多塊網路卡及IP來連線，這樣的話，就構成了多對多的關係。也就是說，主機到儲存可以有多條路徑可以選擇。主機到儲存之間的IO由多條路徑可以選擇。每個主機到所對應的儲存可以經過幾條不同的路徑，如果是同時使用的話，I/O流量如何分配？其中一條路徑壞掉了，如何處理？從在作業系統的角度來看，每條路徑，作業系統會認為是一個實際存在的物理盤，但實際上只是通向同一個物理盤的不同路徑而已，這樣是在使用的時候，就給使用者帶來了困惑。多路徑軟體就是為了解決上面的問題應運而生的。

多路徑的主要功能就是和儲存裝置一起配合實現如下功能：
1、故障的切換和恢復
2、IO流量的負載均衡
3、磁碟的虛擬化

由於多路徑軟體是需要和儲存在一起配合使用的，不同的廠商基於不同的作業系統，都提供了不同的版本。並且有的廠商，軟體和硬體也不是一起賣的，如果要使用多路徑軟體的話，可能還需要向廠商購買license才行。比如EMC公司基於linux下的多路徑軟體，就需要單獨的購買license。好在， RedHat和Suse的2.6的核心中都自帶了免費的多路徑軟體包，並且可以免費使用，同時也是一個比較通用的包，可以支援大多數儲存廠商的裝置，即使是一些不是出名的廠商，透過對配置檔案進行稍作修改，也是可以支援並執行的很好的。

一、Linux下multipath介紹

在redhat中，安裝作業系統的時候multipath已經預設被安裝，檢視安裝情況：

[root@seaing ~]# rpm -qa | grep device-mapper

device-mapper-multipath-0.4.7-34.el5

device-mapper-1.02.39-1.el5

device-mapper-event-1.02.39-1.el5

1、device-mapper-multipath：即multipath-tools。主要提供multipathd和multipath等工具和 multipath.conf等配置檔案。這些工具透過device mapper的ioctr的介面建立和配置multipath裝置（呼叫device-mapper的使用者空間庫。建立的多路徑裝置會在/dev /mapper中）。

2、 device-mapper：主要包括兩大部分：核心部分和使用者部分。核心部分主要由device mapper核心（dm.ko）和一些target driver（md-multipath.ko）。核心完成裝置的對映，而target根據對映關係和自身特點具體處理從mappered device 下來的i/o。同時，在核心部分，提供了一個介面，使用者透過ioctr可和核心部分通訊，以指導核心驅動的行為，比如如何建立mappered device，這些divece的屬性等。linux device mapper的使用者空間部分主要包括device-mapper這個包。其中包括dmsetup工具和一些幫助建立和配置mappered device的庫。這些庫主要抽象，封裝了與ioctr通訊的介面，以便方便建立和配置mappered device。multipath-tool的程式中就需要呼叫這些庫。

3、dm-multipath.ko和dm.ko：dm.ko是device mapper驅動。它是實現multipath的基礎。dm-multipath其實是dm的一個target驅動。

4、scsi_id：包含在udev程式包中，可以在multipath.conf中配置該程式來獲取scsi裝置的序號。透過序號，便可以判斷多個路徑對應了同一裝置。這個是多路徑實現的關鍵。scsi_id是透過sg驅動，向裝置傳送EVPD page80或page83 的inquery命令來查詢scsi裝置的標識。但一些裝置並不支援EVPD 的inquery命令，所以他們無法被用來生成multipath裝置。但可以改寫scsi_id，為不能提供scsi裝置標識的裝置虛擬一個識別符號，並輸出到標準輸出。multipath程式在建立multipath裝置時，會呼叫scsi_id，從其標準輸出中獲得該裝置的scsi id。在改寫時，需要修改scsi_id程式的返回值為0。因為在multipath程式中，會檢查該直來確定scsi id是否已經成功得到。

想了解更多關於Device Mapper Multipath（DM-Multipath），請參考博文：http://blog.csdn.net/holandstone/article/details/7050234

二、Linux下multipath詳細配置

首先，我們本次Linux下multipath的整體的拓撲結構，如下所示：

因為在安裝作業系統的時候，multipath的軟體包已經那裝好了，所以這裡就不在需要安裝，如果沒有安裝的話，需要手動的進行安裝。

1、檢查安裝是否正常

[root@seaing ~]# lsmod |grep dm_multipath

dm_multipath 56921 2 dm_round_robin

scsi_dh 42177 1 dm_multipath

dm_mod 101649 19 dm_multipath,dm_raid45,dm_snapshot,dm_zero,dm_mirror,dm_log

如果模組沒有載入成功，請使用下面的命令初始化DM：

[root@seaing ~]#modprobe dm-multipath
[root@seaing ~]#modprobe dm-round-robin
[root@seaing ~]#service multipathd start
[root@seaing ~]#multipath –v2

2、編輯配置檔案

[root@seaing ~]# cat /etc/multipath.conf

blacklist {

devnode "^sda"

}

defaults {

user_friendly_names no

}

multipaths {

multipath {

wwid 3600508b4000892b90002a00000050000 #磁碟的WWID

alias comsys-dm0 #對映後的別名，自己命名

path_grouping_policy multibus #路徑組策略

path_checker tur #決定路徑狀態的方法

path_selector "round-robin 0" #選擇那一條路徑進行下次IO操作

}

multipath {

wwid 3600508b4000892b90002a00000090000

alias comsys-dm1

path_grouping_policy multibus

path_checker tur

path_selector "round-robin 0"

}

multipath {

wwid 3600508b4000892b90002a00000140000

alias comsys-backup

path_grouping_policy multibus

path_checker tur

path_selector "round-robin 0"

}

devices {

device {

vendor "HP" #廠商名稱，可透過multipath –v3獲取到

product "HSV300" #產品型號

path_grouping_policy multibus #預設的路徑組策略

getuid_callout "/sbin/scsi_id -g -u -s /block/%n" #獲得唯一裝置號使用的預設程式

path_checker readsector0 #決定路徑狀態的方法

path_selector "round-robin 0" #選擇那條路徑進行下一個IO操作的方法

#failback immediate #故障恢復的模式

#no_path_retry queue #在disable queue之前系統嘗試使用失效路徑的次數的數值

#rr_min_io 100 #在當前的使用者組中，在切換到另外一條路徑之前的IO請求的數目

}

備註：

其中 wwid，vendor，product， getuid_callout這些引數可以透過：multipath -v3命令來獲取。如果在/etc/multipath.conf中有設定各wwid別名,別名會覆蓋此設定。

下面是兩種方法來獲取WWID
（1）預設情況下，將使用 /var/lib/multipath/bindings 內的配置設定具體每個多路徑裝置名，如果在/etc/multipath.conf中有設定各wwid 別名，別名會覆蓋此設定。
[root@seaing ~]# cat /var/lib/multipath/bindings
# Multipath bindings, Version : 1.0
# NOTE: this file is automatically maintained by the multipath program.
# You should not need to edit this file in normal circumstances.
#
# Format:
# alias wwid
#
mpath0 SATA_WDC_WD5003ABYX-_WD-WMAYP4135415
mpath1 3600508b4000892b90002a00000050000
mpath2 3600508b4000892b90002a00000090000

（2）透過multipath -v3命令來獲取

[root@seaing ~]# multipath -v3

dm-0: blacklisted

dm-1: blacklisted

dm-2: blacklisted

dm-3: blacklisted

dm-4: blacklisted

loop0: blacklisted

loop1: blacklisted

loop2: blacklisted

loop3: blacklisted

loop4: blacklisted

loop5: blacklisted

loop6: blacklisted

loop7: blacklisted

md0: blacklisted

ram0: blacklisted

ram10: blacklisted

ram11: blacklisted

ram12: blacklisted

ram13: blacklisted

ram14: blacklisted

ram15: blacklisted

ram1: blacklisted

ram2: blacklisted

ram3: blacklisted

ram4: blacklisted

ram5: blacklisted

ram6: blacklisted

ram7: blacklisted

ram8: blacklisted

ram9: blacklisted

sda: blacklisted

sdb: not found in pathvec

sdb: mask = 0x1f

sdb: bus = 1

sdb: dev_t = 8:16

sdb: size = 209715200

sdb: vendor = HP

sdb: product = HSV300

sdb: rev = 0952

sdb: h:b:t:l = 4:0:0:1

sdb: tgt_node_name = 0x50014380013be3f0

sdb: serial = P5512G29SVP02Q

sdb: path checker = readsector0 (controller setting)

sdb: checker timeout = 60000 ms (sysfs setting)

sdb: state = 2

sdb: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sdb: uid = 3600508b4000892b90002a00000050000 (callout)

sdb: getprio = NULL (internal default)

sdb: prio = 1

sdc: not found in pathvec

sdc: mask = 0x1f

sdc: bus = 1

sdc: dev_t = 8:32

sdc: size = 419430400

sdc: vendor = HP

sdc: product = HSV300

sdc: rev = 0952

sdc: h:b:t:l = 4:0:0:2

sdc: tgt_node_name = 0x50014380013be3f0

sdc: serial = P5512G29SVP02Q

sdc: path checker = readsector0 (controller setting)

sdc: checker timeout = 60000 ms (sysfs setting)

sdc: state = 2

sdc: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sdc: uid = 3600508b4000892b90002a00000090000 (callout)

sdc: getprio = NULL (internal default)

sdc: prio = 1

sdd: not found in pathvec

sdd: mask = 0x1f

sdd: bus = 1

sdd: dev_t = 8:48

sdd: size = 209715200

sdd: vendor = HP

sdd: product = HSV300

sdd: rev = 0952

sdd: h:b:t:l = 4:0:0:3

sdd: tgt_node_name = 0x50014380013be3f0

sdd: serial = P5512G29SVP02Q

sdd: path checker = readsector0 (controller setting)

sdd: checker timeout = 60000 ms (sysfs setting)

sdd: state = 2

sdd: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sdd: uid = 3600508b4000892b90002a00000140000 (callout)

sdd: getprio = NULL (internal default)

sdd: prio = 1

sde: not found in pathvec

sde: mask = 0x1f

sde: bus = 1

sde: dev_t = 8:64

sde: size = 209715200

sde: vendor = HP

sde: product = HSV300

sde: rev = 0952

sde: h:b:t:l = 4:0:1:1

sde: tgt_node_name = 0x50014380013be3f0

sde: serial = P5512G29SVP01O

sde: path checker = readsector0 (controller setting)

sde: checker timeout = 60000 ms (sysfs setting)

sde: state = 2

sde: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sde: uid = 3600508b4000892b90002a00000050000 (callout)

sde: getprio = NULL (internal default)

sde: prio = 1

sdf: not found in pathvec

sdf: mask = 0x1f

sdf: bus = 1

sdf: dev_t = 8:80

sdf: size = 419430400

sdf: vendor = HP

sdf: product = HSV300

sdf: rev = 0952

sdf: h:b:t:l = 4:0:1:2

sdf: tgt_node_name = 0x50014380013be3f0

sdf: serial = P5512G29SVP01O

sdf: path checker = readsector0 (controller setting)

sdf: checker timeout = 60000 ms (sysfs setting)

sdf: state = 2

sdf: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sdf: uid = 3600508b4000892b90002a00000090000 (callout)

sdf: getprio = NULL (internal default)

sdf: prio = 1

sdg: not found in pathvec

sdg: mask = 0x1f

sdg: bus = 1

sdg: dev_t = 8:96

sdg: size = 209715200

sdg: vendor = HP

sdg: product = HSV300

sdg: rev = 0952

sdg: h:b:t:l = 4:0:1:3

sdg: tgt_node_name = 0x50014380013be3f0

sdg: serial = P5512G29SVP01O

sdg: path checker = readsector0 (controller setting)

sdg: checker timeout = 60000 ms (sysfs setting)

sdg: state = 2

sdg: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sdg: uid = 3600508b4000892b90002a00000140000 (callout)

sdg: getprio = NULL (internal default)

sdg: prio = 1

sdh: not found in pathvec

sdh: mask = 0x1f

sdh: bus = 1

sdh: dev_t = 8:112

sdh: size = 209715200

sdh: vendor = HP

sdh: product = HSV300

sdh: rev = 0952

sdh: h:b:t:l = 5:0:0:1

sdh: tgt_node_name = 0x50014380013be3f0

sdh: serial = P5512G29SVP02Q

sdh: path checker = readsector0 (controller setting)

sdh: checker timeout = 60000 ms (sysfs setting)

sdh: state = 2

sdh: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sdh: uid = 3600508b4000892b90002a00000050000 (callout)

sdh: getprio = NULL (internal default)

sdh: prio = 1

sdi: not found in pathvec

sdi: mask = 0x1f

sdi: bus = 1

sdi: dev_t = 8:128

sdi: size = 419430400

sdi: vendor = HP

sdi: product = HSV300

sdi: rev = 0952

sdi: h:b:t:l = 5:0:0:2

sdi: tgt_node_name = 0x50014380013be3f0

sdi: serial = P5512G29SVP02Q

sdi: path checker = readsector0 (controller setting)

sdi: checker timeout = 60000 ms (sysfs setting)

sdi: state = 2

sdi: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sdi: uid = 3600508b4000892b90002a00000090000 (callout)

sdi: getprio = NULL (internal default)

sdi: prio = 1

sdj: not found in pathvec

sdj: mask = 0x1f

sdj: bus = 1

sdj: dev_t = 8:144

sdj: size = 209715200

sdj: vendor = HP

sdj: product = HSV300

sdj: rev = 0952

sdj: h:b:t:l = 5:0:0:3

sdj: tgt_node_name = 0x50014380013be3f0

sdj: serial = P5512G29SVP02Q

sdj: path checker = readsector0 (controller setting)

sdj: checker timeout = 60000 ms (sysfs setting)

sdj: state = 2

sdj: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sdj: uid = 3600508b4000892b90002a00000140000 (callout)

sdj: getprio = NULL (internal default)

sdj: prio = 1

sdk: not found in pathvec

sdk: mask = 0x1f

sdk: bus = 1

sdk: dev_t = 8:160

sdk: size = 209715200

sdk: vendor = HP

sdk: product = HSV300

sdk: rev = 0952

sdk: h:b:t:l = 5:0:1:1

sdk: tgt_node_name = 0x50014380013be3f0

sdk: serial = P5512G29SVP01O

sdk: path checker = readsector0 (controller setting)

sdk: checker timeout = 60000 ms (sysfs setting)

sdk: state = 2

sdk: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sdk: uid = 3600508b4000892b90002a00000050000 (callout)

sdk: getprio = NULL (internal default)

sdk: prio = 1

sdl: not found in pathvec

sdl: mask = 0x1f

sdl: bus = 1

sdl: dev_t = 8:176

sdl: size = 419430400

sdl: vendor = HP

sdl: product = HSV300

sdl: rev = 0952

sdl: h:b:t:l = 5:0:1:2

sdl: tgt_node_name = 0x50014380013be3f0

sdl: serial = P5512G29SVP01O

sdl: path checker = readsector0 (controller setting)

sdl: checker timeout = 60000 ms (sysfs setting)

sdl: state = 2

sdl: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sdl: uid = 3600508b4000892b90002a00000090000 (callout)

sdl: getprio = NULL (internal default)

sdl: prio = 1

sdm: not found in pathvec

sdm: mask = 0x1f

sdm: bus = 1

sdm: dev_t = 8:192

sdm: size = 209715200

sdm: vendor = HP

sdm: product = HSV300

sdm: rev = 0952

sdm: h:b:t:l = 5:0:1:3

sdm: tgt_node_name = 0x50014380013be3f0

sdm: serial = P5512G29SVP01O

sdm: path checker = readsector0 (controller setting)

sdm: checker timeout = 60000 ms (sysfs setting)

sdm: state = 2

sdm: getuid = /sbin/scsi_id -g -u -s /block/%n (controller setting)

sdm: uid = 3600508b4000892b90002a00000140000 (callout)

sdm: getprio = NULL (internal default)

sdm: prio = 1

sr0: blacklisted

===== paths list =====

uuid hcil dev dev_t pri dm_st chk_st vend/pr

3600508b4000892b90002a00000050000 4:0:0:1 sdb 8:16 1 [undef][ready] HP,HSV3

3600508b4000892b90002a00000090000 4:0:0:2 sdc 8:32 1 [undef][ready] HP,HSV3

3600508b4000892b90002a00000140000 4:0:0:3 sdd 8:48 1 [undef][ready] HP,HSV3

3600508b4000892b90002a00000050000 4:0:1:1 sde 8:64 1 [undef][ready] HP,HSV3

3600508b4000892b90002a00000090000 4:0:1:2 sdf 8:80 1 [undef][ready] HP,HSV3

3600508b4000892b90002a00000140000 4:0:1:3 sdg 8:96 1 [undef][ready] HP,HSV3

3600508b4000892b90002a00000050000 5:0:0:1 sdh 8:112 1 [undef][ready] HP,HSV3

3600508b4000892b90002a00000090000 5:0:0:2 sdi 8:128 1 [undef][ready] HP,HSV3

3600508b4000892b90002a00000140000 5:0:0:3 sdj 8:144 1 [undef][ready] HP,HSV3

3600508b4000892b90002a00000050000 5:0:1:1 sdk 8:160 1 [undef][ready] HP,HSV3

3600508b4000892b90002a00000090000 5:0:1:2 sdl 8:176 1 [undef][ready] HP,HSV3

3600508b4000892b90002a00000140000 5:0:1:3 sdm 8:192 1 [undef][ready] HP,HSV3

params = 0 0 1 1 round-robin 0 4 1 8:32 1000 8:80 1000 8:128 1000 8:176 1000

status = 2 0 0 0 1 1 A 0 4 0 8:32 A 0 8:80 A 0 8:128 A 0 8:176 A 0

params = 0 0 1 1 round-robin 0 4 1 8:16 1000 8:64 1000 8:112 1000 8:160 1000

status = 2 0 0 0 1 1 A 0 4 0 8:16 A 0 8:64 A 0 8:112 A 0 8:160 A 0

params = 0 0 1 1 round-robin 0 4 1 8:48 1000 8:96 1000 8:144 1000 8:192 1000

status = 2 0 0 0 1 1 A 0 4 0 8:48 A 0 8:96 A 0 8:144 A 0 8:192 A 0

sdb: ownership set to comsys-dm0

sdb: not found in pathvec

sdb: mask = 0xc

sdb: state = 2

sdb: prio = 1

sde: ownership set to comsys-dm0

sde: not found in pathvec

sde: mask = 0xc

sde: state = 2

sde: prio = 1

sdh: ownership set to comsys-dm0

sdh: not found in pathvec

sdh: mask = 0xc

sdh: state = 2

sdh: prio = 1

sdk: ownership set to comsys-dm0

sdk: not found in pathvec

sdk: mask = 0xc

sdk: state = 2

sdk: prio = 1

comsys-dm0: pgfailover = -1 (internal default)

comsys-dm0: pgpolicy = multibus (LUN setting)

comsys-dm0: selector = round-robin 0 (LUN setting)

comsys-dm0: features = 0 (internal default)

comsys-dm0: hwhandler = 0 (internal default)

comsys-dm0: rr_weight = 1 (internal default)

comsys-dm0: minio = 1000 (config file default)

comsys-dm0: no_path_retry = NONE (internal default)

pg_timeout = NONE (internal default)

comsys-dm0: set ACT_NOTHING (map unchanged)

sdc: ownership set to comsys-dm1

sdc: not found in pathvec

sdc: mask = 0xc

sdc: state = 2

sdc: prio = 1

sdf: ownership set to comsys-dm1

sdf: not found in pathvec

sdf: mask = 0xc

sdf: state = 2

sdf: prio = 1

sdi: ownership set to comsys-dm1

sdi: not found in pathvec

sdi: mask = 0xc

sdi: state = 2

sdi: prio = 1

sdl: ownership set to comsys-dm1

sdl: not found in pathvec

sdl: mask = 0xc

sdl: state = 2

sdl: prio = 1

comsys-dm1: pgfailover = -1 (internal default)

comsys-dm1: pgpolicy = multibus (LUN setting)

comsys-dm1: selector = round-robin 0 (LUN setting)

comsys-dm1: features = 0 (internal default)

comsys-dm1: hwhandler = 0 (internal default)

comsys-dm1: rr_weight = 1 (internal default)

comsys-dm1: minio = 1000 (config file default)

comsys-dm1: no_path_retry = NONE (internal default)

pg_timeout = NONE (internal default)

comsys-dm1: set ACT_NOTHING (map unchanged)

sdd: ownership set to comsys-backup

sdd: not found in pathvec

sdd: mask = 0xc

sdd: state = 2

sdd: prio = 1

sdg: ownership set to comsys-backup

sdg: not found in pathvec

sdg: mask = 0xc

sdg: state = 2

sdg: prio = 1

sdj: ownership set to comsys-backup

sdj: not found in pathvec

sdj: mask = 0xc

sdj: state = 2

sdj: prio = 1

sdm: ownership set to comsys-backup

sdm: not found in pathvec

sdm: mask = 0xc

sdm: state = 2

sdm: prio = 1

comsys-backup: pgfailover = -1 (internal default)

comsys-backup: pgpolicy = multibus (LUN setting)

comsys-backup: selector = round-robin 0 (LUN setting)

comsys-backup: features = 0 (internal default)

comsys-backup: hwhandler = 0 (internal default)

comsys-backup: rr_weight = 1 (internal default)

comsys-backup: minio = 1000 (config file default)

comsys-backup: no_path_retry = NONE (internal default)

pg_timeout = NONE (internal default)

comsys-backup: set ACT_NOTHING (map unchanged)

下面是相關引數的標準文件的介紹：

Attribute

Description

wwid

Specifies the WWID of the multipath device to which themultipath attributes apply. This parameter is mandatory for this section of themultipath.conf file.

alias

Specifies the symbolic name for the multipath device to which themultipath attributes apply. If you are usinguser_friendly_names, do not set this value tompathn; this may conflict with an automatically assigned user friendly name and give you incorrect device node names.

path_grouping_policy

Specifies the default path grouping policy to apply to unspecified multipaths. Possible values include:

failover = 1 path per priority group

multibus = all valid paths in 1 priority group

group_by_serial = 1 priority group per detected serial number

group_by_prio = 1 priority group per path priority value

group_by_node_name = 1 priority group per target node name

path_selector

Specifies the default algorithm to use in determining what path to use for the next I/O operation. Possible values include:

round-robin 0: Loop through every path in the path group, sending the same amount of I/O to each.

queue-length 0: Send the next bunch of I/O down the path with the least number of outstanding I/O requests.

service-time 0: Send the next bunch of I/O down the path with the shortest estimated service time, which is determined by dividing the total size of the outstanding I/O to each path by its relative throughput.

failback

Manages path group failback.

A value of immediate specifies immediate failback to the highest priority path group that contains active paths.

A value of manual specifies that there should not be immediate failback but that failback can happen only with operator intervention.

A value of followover specifies that automatic failback should be performed when the first path of a path group becomes active. This keeps a node from automatically failing back when another node requested the failover.

A numeric value greater than zero specifies deferred failback, expressed in seconds.

prio

Specifies the default function to call to obtain a path priority value. For example, the ALUA bits in SPC-3 provide an exploitableprio value. Possible values include:

const: Set a priority of 1 to all paths.

emc: Generate the path priority for EMC arrays.

alua: Generate the path priority based on the SCSI-3 ALUA settings.

tpg_pref: Generate the path priority based on the SCSI-3 ALUA settings, using the preferred port bit.

ontap: Generate the path priority for NetApp arrays.

rdac: Generate the path priority for LSI/Engenio RDAC controller.

hp_sw: Generate the path priority for Compaq/HP controller in active/standby mode.

hds: Generate the path priority for Hitachi HDS Modular storage arrays.

no_path_retry

A numeric value for this attribute specifies the number of times the system should attempt to use a failed path before disabling queueing.

A value of fail indicates immediate failure, without queueing.

A value of queue indicates that queueing should not stop until the path is fixed.

rr_min_io

Specifies the number of I/O requests to route to a path before switching to the next path in the current path group. This setting is only for systems running kernels older that 2.6.31. Newer systems should userr_min_io_rq. The default value is 1000.

rr_min_io_rq

Specifies the number of I/O requests to route to a path before switching to the next path in the current path group, using request-based device-mapper-multipath. This setting should be used on systems running current kernels. On systems running kernels older than 2.6.31, use rr_min_io. The default value is 1.

rr_weight

If set to priorities, then instead of sending rr_min_iorequests to a path before calling path_selector to choose the next path, the number of requests to send is determined byrr_min_io times the path's priority, as determined by theprio function. If set to uniform, all path weights are equal.

flush_on_last_del

If set to yes, then multipath will disable queueing when the last path to a device has been deleted.

3、啟動multipathd服務並設定其自動啟動

[root@seaing ~]#service multipathd restart

[root@seaing ~]#chkconfig --level 345 multipathd on

[root@seaing ~]#chkconfig --list | grep multipathd

4、檢查multipath聚合後的裝置名，以及裝置對應的鏈路情況

[root@seaing ~]# multipath -ll

comsys-dm1 (3600508b4000892b90002a00000090000) dm-3 HP,HSV300

[size=200G][features=0][hwhandler=0][rw]

\_ round-robin 0 [prio=4][active]

\_ 4:0:0:2 sdc 8:32 [active][ready]

\_ 4:0:1:2 sdf 8:80 [active][ready]

\_ 5:0:0:2 sdi 8:128 [active][ready]

\_ 5:0:1:2 sdl 8:176 [active][ready]

comsys-dm0 (3600508b4000892b90002a00000050000) dm-2 HP,HSV300

[size=100G][features=0][hwhandler=0][rw]

\_ round-robin 0 [prio=4][active]

\_ 4:0:0:1 sdb 8:16 [active][ready]

\_ 4:0:1:1 sde 8:64 [active][ready]

\_ 5:0:0:1 sdh 8:112 [active][ready]

\_ 5:0:1:1 sdk 8:160 [active][ready]

comsys-backup (3600508b4000892b90002a00000140000) dm-4 HP,HSV300

[size=100G][features=0][hwhandler=0][rw]

\_ round-robin 0 [prio=4][active]

\_ 4:0:0:3 sdd 8:48 [active][ready]

\_ 4:0:1:3 sdg 8:96 [active][ready]

\_ 5:0:0:3 sdj 8:144 [active][ready]

\_ 5:0:1:3 sdm 8:192 [active][ready]

5、檢查配置是否成功

如果配置正確的話就會在/dev/mapper/目錄下多出mpath0、mpath1（自動分配的名稱）等之類裝置，但是因為作者在配置的時候，起了別名，所以會使用別名來代替自動分配的名稱

[root@seaing ~]# ll /dev/mapper/

total 0

brw-rw---- 1 root disk 253, 4 Oct 26 14:30 comsys-backup

brw-rw---- 1 root disk 253, 2 Oct 26 14:30 comsys-dm0

brw-rw---- 1 root disk 253, 3 Oct 26 14:30 comsys-dm1

crw------- 1 root root 10, 63 Oct 26 14:30 control

brw-rw---- 1 root disk 253, 0 Oct 26 14:30 VolGroup00-LogVol00

brw-rw---- 1 root disk 253, 1 Oct 26 14:30 VolGroup00-LogVol01

用fdisk -l命令可以看到多路徑軟體建立的磁碟，如下圖中的/dev/dm-[0-3]

[root@seaing ~]# fdisk -l

Disk /dev/sda: 500.1 GB, 500107862016 bytes

255 heads, 63 sectors/track, 60801 cylinders