Sometimes, to troubleshoot some multipath related issues, you would require to map the /dev/mapper/mpathY device to its corresponding /dev/sdX device. The /dev/mapper/mpathY is the multipath’d device where as the /dev/sdX is the actual device underlying it. Follow the steps below to find the mapping between the two and vice versa. We would also see how to find the /dev/sdX device name from the corresponding /dev/mapper/dm-* entry.
How to map /dev/mapper/mpathY device from /dev/dm-Z device
1. To determine which /dev/mapper entry corresponds to this /dev/dm-* entries, run the following ls command:
# ls -l /dev/dm-9 brw-r----- 1 root disk 252, 9 Oct 16 00:54 /dev/dm-9
2. Now, examine the friendly names for these devices.
# ls -l /dev/mapper/* brw-rw---- 1 root disk 252, 9 Oct 16 00:54 /dev/mapper/backup brw-rw---- 1 root disk 252, 10 Oct 16 00:54 /dev/mapper/home brw-rw---- 1 root disk 252, 12 Oct 16 00:54 /dev/mapper/log brw-rw---- 1 root disk 252, 11 Oct 16 00:54 /dev/mapper/stage
3. The fifth and sixth fields are the device major and minor numbers. This pair uniquely identifies each device.
4. For the /dev/dm-9 query, we find the pair 252,9 as the system device. By examining the listing of the friendly names we can see that 252,9 pair associated with the /dev/mapper/backup name.
How to map /dev/sdX device from /dev/mapper/ device
To determine the /dev/sdX Device details from the output of /dev/mapper, follow the example shown below.
1. You have a /dev/mapper device as shon below from the ‘df-hP’ command output.
# df -hP Filesystem Size Used Avail Use% Mounted on /dev/mapper/vg_os-lv_os 184G 125G 51G 72% /
2. Run the command below on the /dev/mapper/vg_os-lv_os device as shown below.
# dmsetup table /dev/mapper/vg_os-lv_os 0 392183808 linear 8:2 2048
The fourth field (8:2) is the one which is of interest here.
3. Now run the below command to list all sdX device under /dev folder. For example:
# ls -Al /dev/sd* brw-rw---- 1 root disk 8, 0 Sep 8 10:51 /dev/sda brw-rw---- 1 root disk 8, 1 Mar 30 2017 /dev/sda1 brw-rw---- 1 root disk 8, 2 Mar 30 2017 /dev/sda2 brw-rw---- 1 root disk 8, 3 Mar 30 2017 /dev/sda3
4. The entry 8 (major), 4 (minor) from the “ls -Al” output is the one which matches the 8:2 entry from the “dmsetup table” command. So, /dev/mapper/vg_os-lv_os actually is /dev/sda2.
Using the command blkid and lsblk
The commands blkid and lsblk are very handy to map the relationship between /dev/sdX, /dev/mapper and /dev/dm-* devices. Below are sample outputs of both the commands.
# blkid /dev/mapper/vg_os-lv_root: UUID="d5b718a4-0c7d-4289-b0ae-5787c0248413" TYPE="ext4" /dev/mapper/vg_os-lv_swap: UUID="f578caa6-18b0-46b9-bfc4-10c23aacca01" TYPE="swap"
# lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 838.3G 0 disk ├─sda1 8:1 0 1G 0 part /boot └─sda2 8:2 0 837.3G 0 part ├─vg_os-lv_root (dm-0) 253:0 0 100G 0 lvm / └─vg_os-lv_swap (dm-1) 253:1 0 192G 0 lvm [SWAP]
As seen from the outputs above, /dev/dm-0 corresponds to the root device (/) which in turn maps to /dev/mapper/vg_os-lv_root device.
