HowTo Configure Linux LVM with Multipath Enabled

In this article, your going to learn HowTo Configure Linux LVM Multipath Enabled. Before We published about What is LVM (Logical Volume Manager). Creating LVM itself you have the flexibility to increase/decrease disk space as per the requirement, but why we have to create Multipath for SAN Disks.?

Actually, DM Multipath is used to fail over when one path is failed, it will use another path to send application I/O through multipath (Active/Passive). Another way of using is load sharing when one path is full with I/O it redirect the traffic with another path (Active/Active)

 What is Multipath and How it will be useful

 

As shown in above diagram, Linux server is connected to SAN Storage with dual paths (path-1 and path-2). If you map an LUN from SAN Device it will show in Linux server as two disks because it is coming from two HBA cards (typically two paths).  Here our goal is to combine both the paths and use them as single path (redundancy).

Always do not trust (dependent) single cable connectivity because 

• If FC Cable Fails.?
• HBA Card Failure.?
• SAN Switch Failed.?

If any one the component fails, you loose Disk from your Linux Server as a result hosted application will go down. Now do you understand how CRITICAL is SAN Disk.

Above is the diagram which shows Data flow from Applications to LUN/SAN Storage after multipath configuration

HowTo Install and configure Multipath RHEL 7

To configure multipath in Linux few packages you have to install

# yum install -y device-mapper*

Verify installed packages using rpm command

# rpm -qa |grep device-mapper
device-mapper-persistent-data-0.6.3-1.el7.x86_64
device-mapper-multipath-0.4.9-99.el7.x86_64
device-mapper-libs-1.02.135-1.el7.x86_64
device-mapper-event-1.02.135-1.el7.x86_64
device-mapper-1.02.135-1.el7.x86_64
device-mapper-event-libs-1.02.135-1.el7.x86_64
device-mapper-multipath-libs-0.4.9-99.el7.x86_64

Now do not start multipathd service, we have to copy the multipath config file from /usr/share/doc/device-mapper-multipath-0.4.9/multipath.conf location to /etc/multipath.conf Or you can also generate using simple multipath command

[root@ArkItServ yum.repos.d]# systemctl start multipathd
[root@ArkItServ yum.repos.d]# systemctl status multipathd.service 
● multipathd.service - Device-Mapper Multipath Device Controller
 Loaded: loaded (/usr/lib/systemd/system/multipathd.service; enabled; vendor preset: enabled)
 Active: inactive (dead)
Condition: start condition failed at Thu 2017-08-17 19:04:38 IST; 7s ago
 ConditionPathExists=/etc/multipath.conf was not met

Generate Multipath Configuration file using below command and start multipathd service

# mpathconf --enable --user_friendly_names y

[root@ArkItServ ~]# modprobe dm_multipath
[root@ArkItServ ~]# lsmod |grep dm_multipath
dm_multipath 23065 0 
dm_mod 114430 23 dm_multipath,dm_log,dm_persistent_data,dm_mirror,dm_bufio,dm_thin_pool

HowTo Configure Linux LVM Multipath

Map the LUN from SAN Storage to Linux Server using FCP protocol

Scan for New devices using below command

ls /sys/class/scsi_host/ | while read host ; do echo "- - -" > /sys/class/scsi_host/$host/scan ; done

Now verify multipath disks automatically get friendly name you already enabled in multipath configuration

If you exclude few devices list by specifying block list they will not show in multipath command list

Use multipath alias section to specify alternative name to override friendly name. Below is the actual /etc/multupath.conf file

### Default Multipath configuration RHEL 7 ###
defaults {
 udev_dir /dev
 polling_interval 10
 selector "round-robin 0"
 path_grouping_policy failover
 getuid_callout "/sbin/scsi_id -g -u -s /block/%n"
 # prio_callout "/sbin/mpath_prio_ontap /dev/%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
}

## Device Blacklist Rule ##
blacklist {
 devnode "sd[a]$"
 devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
 devnode "^hd[a-z]"
 devnode "^cciss!c[0-9]d[0-9]*[p[0-9]*]"

}

## Multipath Aliases ###

multipaths {

multipath {
 wwid 4500a098038303635685d472f492d6b5q
 alias DATA-DISK
 path_grouping_policy failover

# ONTAP Path: ark-netapp-03:/vol/prod_ora_disk1/prod_disk1
 # LUN: 1
 # LUN Size: 500g
 # Host Device: mpath1(4500a098038303635685d472f492d6b5q)
 # 5:0:0:5 sdp
 # 6:0:0:5 sdq
 # 6:0:1:5 sdaa
 # 5:0:1:5 sdr
 }
}

Now check new changes using multipath -v2 command

• -v level  
• verbosity, print all paths and multipaths
• 0 no output
• 1 print the created or updated multipath names only, for use to feed other tools like kpartx
• 2 + print all info : detected paths, coalesced paths (ie multipaths) and device maps

Add Partition Mappings using kpartx command

# kpartx -a /dev/mapper/DATA-DISK

Verify the device mapper

# ls -al /dev/mapper/

Create Linux LVM partition using fdisk utility

# fdisk /dev/mapper/DATA-DISK

:n
:p
Enter
Enter
t
8e
:wq

Start Creating Linux LVM using multipath disk

# pvcreate /dev/mapper/DATA-DISKp1
 Writing physical volume data to disk "/dev/mapper/DATA-DISKp1"
 Physical volume "/dev/mapper/DATA-DISKp1" successfully created

Physical volume is created successfully, using multipath Linux LVM

# vgcreate DATAVG /dev/mapper/DATA-DISKp1
 Volume group "DATAVG" successfully created

Volume Group has created using Multipath PV

# lvcreate -n LV03 -l 100%FREE DATAVG
 Logical volume "LV03" created

Logical Volume with 100% free space now in place.

Format Newly created Logical Volume using below command

# mkfs -t ext4 /dev/DATAVG/LV03

You have successfully configure Linux LVM Multipath

Mount formatted partition and use it

# mount /dev/DATAVG/LV03 /mountpoint

Permanent mount add entry in fstab file

/dev/DATAVG/LV03                           /mountpoint                    ext4    defaults        1 2

That’s All we achieved is redundant disk path in case of one path failure.

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