How to create partitions and file systems on DM-Multipath devices?

Device-Mapper Multipath (DM-Multipath) is a Linux native multipath tool, which allows you to configure multiple I/O paths between server nodes and storage arrays into a single device. These I/O paths are physical SAN connections that can include separate cables, switches, and controllers. Multipathing aggregates the I/O paths, creating a new device that consists of the aggregated paths.

 

Create partitions DM-Multipath devices

 

The DM-Multipath devices will be created as /dev/mapper/mpathN, where N is the multipath group number.

 

1. Use command fdisk to create partitions on /dev/mapper/mpathN. Use the options “n” to create a new partition and provide “p” as the partition type which statnds for primary partition.

# fdisk /dev/mapper/mpath0

Command (m for help): n   
Command action
   e   extended
   p   primary partition (1-4)
p

 

2. Provide the partition number, first cylider (we will use the default value of 1) and last cylinder or size of the partition.

Partition number (1-4): 1
First cylinder (1-1017, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-1017, default 1017):
Using default value 1017

 

3. Use the options “w” to write the partition table from memory to disk.

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.

 

4. Register multipath partitions in /dev/mapper using the “kpartx” command:

# kpartx -a /dev/mapper/mpath0

 

5. List all partitions on this device:

# kpartx -l /dev/mapper/mpath0
mpath0p1 : 0 2295308 /dev/mapper/mpath0 61

 

Creating a filesystem on DM-Multipath device partition

 

1. Finally, we can now create a filesystem on the newly created partition on the DM-multipath device.

# mkfs.ext4 /dev/mapper/mpath0p1
mke2fs 1.42.9 (28-Dec-2013)

Filesystem too small for a journal
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
Stride=0 blocks, Stripe width=0 blocks
64 inodes, 508 blocks
25 blocks (4.92%) reserved for the super user
First data block=1
Maximum filesystem blocks=524288
1 block group
8192 blocks per group, 8192 fragments per group
64 inodes per group

Allocating group tables: done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done

 

2. Create a directory and mount the partition on the mount point:

# mkdir /data
# mount /dev/mapper/mpath0p1 /data

 

Notes :

The DM-Multipath tool uses three different sets of file names:


1. /dev/dm-N
2. /dev/mpath/mpathN
3. /dev/mapper/mpathN

 

NEVER use /dev/dm-N devices,as they are only intended to be used by the DM-Multipath tool. NEVER use /dev/mpath/mpathN devices, because when multipath devices are mounted at boot time, the UDEV subsystem may not create the device nodes soon enough.

 

ALWAYS use /dev/mapper/mpathN devices, as they are persistent and they are automatically created by device-mapper early in the boot process. Therefore these are the device names that should be used to access the multipathed devices. But in a RAC (Real Application Clustrer) configuration, although /dev/mapper/mpathN names may be persistent across reboots on a single machine, there is no guarantee that other cluster nodes will use the same name for this disk. If this is desired, then use the UDEV facility to get cluster-wide persistent names.

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