Linux Multipath command is used to manage storage SAN (storage area network) disks on OS side. Linux multipath provides a way of organizing the I/O paths logically, by creating a single multipath device on top of the underlying devices.
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.
We use our Pure Storage as an example.
Table of Contents
How to start multipath in Linux?
To make multipath work, we need make sure below module is loaded
# lsmod | grep dm_multipath
dm_multipath 27427 4 dm_round_robin,dm_service_time
# modinfo dm_multipath
filename: /lib/modules/3.10.0-693.21.1.el7.x86_64/kernel/drivers/md/dm-multipath.ko.xz
license: GPL
author: Sistina Software
description: device-mapper multipath target
retpoline: Y
rhelversion: 7.4
srcversion: 063067F9F167E7B653A4773
depends: dm-mod
intree: Y
vermagic: 3.10.0-693.21.1.el7.x86_64 SMP mod_unload modversions
signer: Red Hat Enterprise Linux kernel signing key
sig_key: B1:C3:31:09:FB:DA:94:AD:2F:E1:E8:E5:C1:E5:52:BD:22:57:60:FE
sig_hashalgo: sha256
multipath needs these two rpms.
- device-mapper-multipath
- device-mapper
start multipathd service with service command or systemctl command
List the connected and scanned multipath devices on Linux
# multipath -l
Each multipath device has a World Wide Identifier (WWID), which is guaranteed to be globally unique and unchanging.
You can get the WWID using the below commands.
# multipath -l | grep dm
3624a937079ebbf3c903141360001149b dm-5 PURE ,FlashArray
3624a937079ebbf3c903141360001149a dm-14 PURE ,FlashArray
OR using below command
# ls -ld /dev/disk/by-id/scsi-*
lrwxrwxrwx 1 root root 9 May 22 18:29 /dev/disk/by-id/scsi-3600c0ff00013b88f9058e35a01000000 -> ../../sde
lrwxrwxrwx 1 root root 9 May 22 18:29 /dev/disk/by-id/scsi-3600c0ff00013b88f9c58e35a01000000 -> ../../sdd
2 ways to Get the WWID ( SCSI ID ) of the disks on Linux
Each LUN is mapped to a disk for which also a unique WWID is generated and the same can be used for representing them in the configuration as the device name may change post reboot while the WWID will not change and are reboot persistent.
# lsscsi --scsi_id
3624a937079ebbf3c903141360001149c dm-2 PURE ,FlashArray
size=1.0T features='0' hwhandler='0' wp=rw
`-+- policy='queue-length 0' prio=1 status=active
|- 13:0:0:249 sdo 8:224 active ready running
|- 11:0:0:249 sdr 65:16 active ready running
|- 11:0:1:249 sdas 66:192 active ready running
`- 13:0:1:249 sdan 66:112 active ready running
To get the WWID of the disks you can also use the below command
# /usr/lib/udev/scsi_id -g -u -d /dev/sda
3600c0ff00013b88f9058e35a01000000
2 ways to list of disks mapped via HBA on Linux box
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.
For our case since we have 1 LUN connected to the Linux box hence we see 4 devices
# ls -ld /sys/block/sd*/device
lrwxrwxrwx 1 root root 0 May 22 18:29 /sys/block/sda/device -> ../../../0:0:0:0
lrwxrwxrwx 1 root root 0 May 22 18:29 /sys/block/sdb/device -> ../../../0:0:0:1
lrwxrwxrwx 1 root root 0 May 22 18:29 /sys/block/sdc/device -> ../../../0:0:1:0
lrwxrwxrwx 1 root root 0 May 22 18:29 /sys/block/sdd/device -> ../../../0:0:1:1
The same can be seen via below command where LUN to device mapping does easier on the eyes
# ls -l /dev/disk/by-path/
total 0
lrwxrwxrwx 1 root root 9 May 22 18:29 pci-0000:04:00.2-fc-0x207000c0ff13d3a7-lun-0 -> ../../sda
lrwxrwxrwx 1 root root 9 May 22 18:29 pci-0000:04:00.2-fc-0x207000c0ff13d3a7-lun-1 -> ../../sdb
lrwxrwxrwx 1 root root 9 May 22 18:29 pci-0000:04:00.2-fc-0x247000c0ff13d3a7-lun-0 -> ../../sdc
lrwxrwxrwx 1 root root 9 May 22 18:29 pci-0000:04:00.2-fc-0x247000c0ff13d3a7-lun-1 -> ../../sdd
How does Device ID work in multipath?
In multipath output we see below highlighted entry being used (and also in the commands above you would see similar representation)
# multipath -l
3624a937079ebbf3c903141360001149c dm-9 PURE ,FlashArray
size=1.0T features='0' hwhandler='0' wp=rw
`-+- policy='queue-length 0' prio=1 status=active
|- 11:0:0:249 sdo 8:224 active ready running
|- 11:0:1:249 sdao 66:128 active ready running
|- 13:0:0:249 sdr 65:16 active ready running
`- 13:0:1:249 sdas 66:192 active ready running
Below value explains what each field means
1:0:0:1
^ ^ ^ ^
| | | |
H C T L
- H is the HBA number
- C is the channel on the HBA
- T is the SCSI target ID
- L is the LUN from the Storage
Flush/Remove all the multipath device on Linux
# multipath -F
Flush/Remove selected multipath device
First of all the the WWID of the multipath device which you want to flush using multipath -ll
# multipath -ll
3600c0ff00013b88fdc53e35a01000000 dm-0 PURE ,P2000 G3 FC
size=186G features='1 queue_if_no_path' hwhandler='0' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| |- 1:0:0:0 sda 8:0 active ready running
| `- 2:0:1:0 sdd 8:48 active ready running
`-+- policy='service-time 0' prio=10 status=enabled
|- 1:0:1:0 sdb 8:16 active ready running
`- 2:0:0:0 sdc 8:32 active ready running
Next flush the selected multipath using below command
# multipath -f 3600c0ff000144876065be35a01000000
NOTE: This multipath must not be in used state
Show the existing multipath configuration in detail on Linux
Below command will show the existing configuration loaded on your Linux box
# multipathd show config
How to Get complete HBA details on Linux
systool uses APIs provided by libsysfs to gather information and will give detailed information about the available HBA and the LUN status
NOTE: systool is delivered with sysfsutils rpm so make sure it is installed before using this tool.
# systool -c fc_host -v
How to get the HBA port name on Linux
# systool -c fc_host -A port_name
Class = "fc_host"
Class Device = "host0"
port_name = "0x50060b0000c2a67c"
Device = "host0"
Class Device = "host1"
port_name = "0x50060b0000c2a67e"
Device = "host1"
To get the port id
# systool -c fc_host -A port_id
Class = "fc_host"
Class Device = "host0"
port_id = "0x310305"
Device = "host0"
Class Device = "host1"
port_id = "0x25030f"
Device = "host1"
Show the existing multipath configuration in brief
This is an alternate command which can be used to check the loaded configuration for selected values as shown below.
# mpathconf
multipath is enabled
find_multipaths is disabled
user_friendly_names is enabled
dm_multipath module is loaded
multipathd is running
Each multipath device has a World Wide Identifier (WWID), which is guaranteed to be globally unique and unchanging. By default, the name of a multipath device is set to its WWID.
Alternately, you can set the user_friendly_names option in the multipath configuration file, which sets the alias to a nodeunique name of the form mpathn
With user_friendly name disabled we will only see the WWID of the multipath connected
# multipath -l
3624a937079ebbf3c903141360001149c dm-2 PURE ,FlashArray
size=1.0T features='0' hwhandler='0' wp=rw
`-+- policy='queue-length 0' prio=1 status=active
|- 13:0:0:249 sdo 8:224 active ready running
|- 11:0:0:249 sdr 65:16 active ready running
|- 11:0:1:249 sdas 66:192 active ready running
`- 13:0:1:249 sdan 66:112 active ready running
Next enable user_friendly name
# /sbin/mpathconf --user_friendly_names y
Check the multipath
# multipath -l
mpathb (3600c0ff000144876065be35a01000000) dm-6 HP ,P2000 G3 FC
size=359G features='1 queue_if_no_path' hwhandler='0' wp=rw
|-+- policy='service-time 0' prio=0 status=active
| `- 2:0:0:1 sdb 8:16 active undef unknown
`-+- policy='service-time 0' prio=0 status=enabled
`- 2:0:1:1 sdd 8:48 active undef unknown
The mapping is done using /etc/multipath/bindings.
how to Check the HBA port name on your Linux box
# ls -l /sys/class/fc_host/
total 0
lrwxrwxrwx 1 root root 0 May 22 18:29 host0 -> ../../devices/pci0000:00/0000:00:02.0/0000:04:00.2/host0/fc_host/host0
lrwxrwxrwx 1 root root 0 May 22 18:29 host1 -> ../../devices/pci0000:00/0000:00:02.0/0000:04:00.3/host1/fc_host/host1
You can also check the same using
# lspci -nn |grep -i "Fibre"
04:00.2 Fibre Channel [0c04]: Emulex Corporation OneConnect 10Gb FCoE Initiator (be3) [19a2:0714] (rev 01)
04:00.3 Fibre Channel [0c04]: Emulex Corporation OneConnect 10Gb FCoE Initiator (be3) [19a2:0714] (rev 01)
On some Linux variant, you can also grep for HBA
# lspci -nn |grep -i "HBA"
How to Get the HBA WWPN on Linux?
A worldwide port number (WWPN) is unique identifier for each FC port of any Fibre Channel device.
- For server, we have a WWPN for each port of the HBA.
- For a SAN switch, the WWPN is available for each port in the chassis.
- For Storage, each host port has an individual WWPN
Here we have two HBA so each HBA will have unique World Wide Port Name (WWPN) which will be connected to the SAN switch then to the Storage Box.
From my Linux Box
# cat /sys/class/fc_host/host0/port_name
0x50060b0000c2a67c
# cat /sys/class/fc_host/host1/port_name
0x50060b0000c2a67e
These are the WWPN number.
How to Get the HBA WWNN on Linux?
A worldwide node name (WWNN) is a globally unique 64-bit identifier that is assigned to each Fibre Channel node or device.
- For servers and hosts, WWNN is unique for each HBA (host bus adapter), and in a case of a server with two HBAs, they have two WWNNs.
- For a SAN switch, the WWNN is a common for the chassis.
- For storage, the WWNN is common for each controller unit of midrage storage
Since here I have two HBA hence there are two WWNN for each HBA
From my Linux Box
# cat /sys/class/fc_host/host1/node_name
0x50060b0000c2a67f
# cat /sys/class/fc_host/host0/node_name
0x50060b0000c2a67d