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I cannot figure out the mapping between different logical and physical block device names. The output of "cat /proc/diskstats" is :

 104    0 cciss/c0d0 ...
 104    1 cciss/c0d0p1 ...
 104    2 cciss/c0d0p2 ...
 104    16 cciss/c0d1 ...
 253    0 dm-0 ...
 253    1 dm-1 ...
 253    2 dm-2 ...
 253    3 dm-3 ...

The output of "df -h" is :

/dev/cciss/c0d0p1                 99M   39M   56M  42% /boot
/dev/mapper/VolGroup01-LogVol02   908G  760G  103G  89% /home
/dev/mapper/VolGroup01-LogVol03   193G  181G  2.6G  99% /opt
/dev/mapper/VolGroup00-LogVol00   54G   11G   41G  21%  /

Where do I find the mapping between "cciss" , "dm-#", and "VolGroup##" ?

Thanks

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1
  • 3
    Everything in the /dev/mapper directory is typically a symlink to the actual device.
    – jw013
    Jun 19, 2012 at 1:29

7 Answers 7

Reset to default
30

There're handy dmsetup ls --tree and lsblk utils.

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3
  • 2
    Although that doesn't give you the mapping between VGs and their PVs. For that, you'd need to run pvdisplay and vgdisplay and correlate their output to that of dmsetup ls.
    – Alexios
    Jun 19, 2012 at 9:54
  • 3
    @Alexios, ? Didn't try lsblk?
    – poige
    Jun 20, 2012 at 13:17
  • No lsblk on CentOS 5.
    – karatedog
    Sep 25, 2019 at 17:05
7

I was just running into this, so I'll document what helped me here.

poise's answer is correct, You can get all of the information you need from dmsetup ls --tree, if you know how to interpret the output.

cciss is the device name, that's your actual disk. The man page spells it out well, but I'll copy the relevant section here:

   Device nodes
   The device naming scheme is as follows:

   Major numbers:

       104     cciss0
       105     cciss1
       106     cciss2
       105     cciss3
       108     cciss4
       109     cciss5
       110     cciss6
       111     cciss7

   Minor numbers:

       b7 b6 b5 b4 b3 b2 b1 b0
       |----+----| |----+----|
            |           |
            |           +-------- Partition ID (0=wholedev, 1-15 partition)
            |
            +-------------------- Logical Volume number

   The device naming scheme is:

       /dev/cciss/c0d0         Controller 0, disk 0, whole device
       /dev/cciss/c0d0p1       Controller 0, disk 0, partition 1
       /dev/cciss/c0d0p2       Controller 0, disk 0, partition 2
       /dev/cciss/c0d0p3       Controller 0, disk 0, partition 3

       /dev/cciss/c1d1         Controller 1, disk 1, whole device
       /dev/cciss/c1d1p1       Controller 1, disk 1, partition 1
       /dev/cciss/c1d1p2       Controller 1, disk 1, partition 2
       /dev/cciss/c1d1p3       Controller 1, disk 1, partition 3

The "dm-#" is the device mapper number. The easiest way to map DM numbers is to run lvdisplay, which shows the logical volume name, the volume group it belongs to, and the block device. In the "Block device" row, the value listed after the colon is the DM number. 

root@centos:/dev > lvdisplay /dev/vg0/opt 
  --- Logical volume ---
  LV Name                /dev/vg0/opt
  VG Name                vg0
  LV UUID                ObffAT-txIn-5Rwy-bW5s-gekn-VLZv-71mDZi
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                1.00 GB
  Current LE             32
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:5

Which maps back nicely to the output of dmsetup ls --tree

vg0-opt (253:5)
 └─ (104:3)

You can also see the DM number mappings by running ls -lrt /dev/mapper.

root@centos:/dev > ls -lrt /dev/mapper
total 0
crw------- 1 root root  10, 60 Aug 29  2013 control
brw-rw---- 1 root disk 253,  0 Aug 29  2013 vg0-root
brw-rw---- 1 root disk 253,  1 Aug 29  2013 vg0-usr
brw-rw---- 1 root disk 253,  2 Aug 29  2013 vg0-tmp
brw-rw---- 1 root disk 253,  3 Aug 29  2013 vg0-var
brw-rw---- 1 root disk 253,  4 Aug 29  2013 vg0-home
brw-rw---- 1 root disk 253,  5 Aug 29  2013 vg0-opt

The sixth column lists the DM number. So, for my server, vg0-opt is mounted on /opt, and maps back to DM-5.

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6

Use dmsetup ls --tree -o blkdevname.

[root@redhat7 Packages]# dmsetup ls --tree -o blkdevname
rhel-swap <dm-1> (253:1)
 └─ <sda2> (8:2)
rhel-root <dm-0> (253:0)
 └─ <sda2> (8:2)
oraclevg-oraclelv <dm-2> (253:2)
 └─ <sdb> (8:16)
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1
  • needs root access
    – iruvar
    Nov 29, 2019 at 19:39
3

Ok, finally figured that out.

pvscan command provides the mapping I'm looking for.

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3

@Jon Buys's and @poige's answers are correct, but at least on systems with a relatively recent kernel, it is even simpler.

The entries in /dev/mapper are simply soft links pointing to the dm-x device in /dev, so all you should need is:

ls -l /dev/mapper

For example, on my system, this shows:

# ls -l /dev/mapper/  
total 0
crw-------. 1 root root 10, 236 Dec 12 10:11 control
lrwxrwxrwx. 1 root root       7 Dec 12 10:11 vg_raid10-lv_cache_pool_home_cdata -> ../dm-3
lrwxrwxrwx. 1 root root       7 Dec 12 10:11 vg_raid10-lv_cache_pool_home_cmeta -> ../dm-4
lrwxrwxrwx. 1 root root       7 Dec 12 10:11 vg_raid10-lv_cache_pool_var_cdata -> ../dm-7
lrwxrwxrwx. 1 root root       7 Dec 12 10:11 vg_raid10-lv_cache_pool_var_cmeta -> ../dm-8
lrwxrwxrwx. 1 root root       7 Dec 12 10:11 vg_raid10-lv_home -> ../dm-6
lrwxrwxrwx. 1 root root       7 Dec 12 10:11 vg_raid10-lv_home_corig -> ../dm-5
lrwxrwxrwx. 1 root root       8 Dec 12 10:11 vg_raid10-lv_var -> ../dm-10
lrwxrwxrwx. 1 root root       7 Dec 12 10:11 vg_raid10-lv_var_corig -> ../dm-9
lrwxrwxrwx. 1 root root       7 Dec 12 10:11 vg_ssd-lv_root -> ../dm-0
lrwxrwxrwx. 1 root root       7 Dec 12 10:11 vg_ssd-lv_swap -> ../dm-1
lrwxrwxrwx. 1 root root       8 Dec 27 20:00 vg_ssd_sata-docker--pool -> ../dm-13
lrwxrwxrwx. 1 root root       8 Dec 27 20:00 vg_ssd_sata-docker--pool_tdata -> ../dm-12
lrwxrwxrwx. 1 root root       8 Dec 27 20:00 vg_ssd_sata-docker--pool_tmeta -> ../dm-11
lrwxrwxrwx. 1 root root       7 Dec 27 20:00 vg_ssd_sata-lv_scratch -> ../dm-2
lrwxrwxrwx. 1 root root       8 Dec 27 20:00 vg_ssd_sata-lv_vms -> ../dm-14

If your ls is unhelpful and does not show the soft links, try this:

for x in /dev/mapper/*; do echo "$(realpath $x) -> $x"; done;

which should output something like:

/dev/mapper/control -> /dev/mapper/control
/dev/dm-3 -> /dev/mapper/vg_raid10-lv_cache_pool_home_cdata
/dev/dm-4 -> /dev/mapper/vg_raid10-lv_cache_pool_home_cmeta
...

and if you want to find a specific mapping e.g. dm-6:

# (for x in /dev/mapper/*; do echo "$(realpath $x) -> $x"; done;) | grep dm-6
/dev/dm-6 -> /dev/mapper/vg_raid10-lv_home
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1

I've had the same issue, maybe little "deeper": up to the mount point level. In case anyone is interested, here are two functions I'm using, for querying both ways.

######## FIND THE LVPATH of an existing FS. Query the lvm using FS' mount point
fileSystem_to_lvPath(){
    FS_TO_QUERY=$1
    #Call like this:  $0 /tmp
    #Relevant commands for debug: blkid, lsblk, dmsetup, lvdisplay, lvs
    #OLD Solution: DEV_MAPPER=$(df -l --output=source $1 | awk '{print $1}' | cut -d"/" -f 4 | tail -1)

    #Find DeviceMapper_MajorMinorNumber for specific fs
    DeviceMapper_MajorMinorNumber=$(lsblk --noheadings --output TYPE,MAJ:MIN,MOUNTPOINT | grep -w lvm | grep -w $FS_TO_QUERY | awk '{print $2}')

    #VG=$(lvs --noheadings --separator : --options lv_kernel_major,lv_kernel_minor,vg_name,lv_name,lv_path | grep $DeviceMapper_MajorMinorNumber | awk -F : '{print $3}')
    #LV=$(lvs --noheadings --separator : --options lv_kernel_major,lv_kernel_minor,vg_name,lv_name,lv_path | grep $DeviceMapper_MajorMinorNumber | awk -F : '{print $4}')
    LV_PATH=$(lvs --noheadings --separator : --options lv_kernel_major,lv_kernel_minor,vg_name,lv_name,lv_path | grep $DeviceMapper_MajorMinorNumber | awk -F : '{print $5}')
    echo $LV_PATH
    #echo "$VG/$LV"
}

and the reverse query:

######## FIND THE FS (and FS' mountpoint) of an existing LVPATH:
 lvPath_to_fileSystem(){
    LV_PATH=$1
    #Call like this:  $0 /dev/vg00/opt
    #Relevant commands for debug: blkid, lsblk, dmsetup, lvdisplay, lvs
    #OLD Solution: DEV_MAPPER=$(df -l --output=source $1 | awk '{print $1}' | cut -d"/" -f 4 | tail -1)

    #Find DeviceMapper_MajorMinorNumber for specific lv_path
    DeviceMapper_MajorMinorNumber=$(lvs --noheadings --separator : --options lv_kernel_major,lv_kernel_minor,vg_name,lv_name,lv_path | grep $LV_PATH | awk -F : '{print $1":"$2}')

    FS=$(lsblk --noheadings --output TYPE,MAJ:MIN,MOUNTPOINT | grep -w lvm | grep -w $DeviceMapper_MajorMinorNumber | awk '{print $3}')

    echo $FS
}
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-2

All binding are mapped in file /etc/multipath/bindings. You can rename here any device like from /dev/mapper/mpathf to /dev/mapper/mpathe.

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