[dm-devel] What is a /dev/dm-* file ?

[li mohua]

from the device number(major,minor), we could see it's another kind
for device under /dev/mapper/*,sometimes could be a link(udev rules
etc,no idea about the max number, you could have a test, i guess, 




        [root at 57 mapper]# ls -lrt
        total 0
        crw------- 1 root root  10, 63 Jul 18 05:52 control
        brw-rw---- 1 root disk 253,  0 Jul 18 05:52
        3600a0b80005b10ca00008e254c7726b1
        brw-rw---- 1 root disk 253,  1 Jul 18 05:52
        3600a0b80005b0acc00008c204c7744ba
        brw-rw---- 1 root disk 253,  3 Jul 18 05:52
        3600a0b80005b0acc00008c204c7744bap2
        brw-rw---- 1 root disk 253,  2 Jul 18 05:52
        3600a0b80005b0acc00008c204c7744bap1
        brw-rw---- 1 root disk 253,  4 Jul 18 05:52
        3600a0b80005b10ca00008e254c7726b1p1
        brw-rw---- 1 root disk 253,  5 Jul 18 05:52 HostVG-Swap
        brw-rw---- 1 root disk 253,  6 Jul 18 05:52 HostVG-RootBackup
        brw-rw---- 1 root disk 253,  7 Jul 18 05:52 HostVG-Root
        brw-rw---- 1 root disk 253,  9 Jul 18 05:52 HostVG-Logging
        brw-rw---- 1 root disk 253,  8 Jul 18 05:52 HostVG-Config
        brw-rw---- 1 root disk 253, 10 Jul 18 05:52 HostVG-Data
        brw-rw---- 1 root disk 253, 11 Jul 18 05:52 live-rw
        
        
        
        
        [root at 57 mapper]# ls -lrt /dev/dm*
        brw-rw---- 1 root root 253,  0 Jul 18 05:52 /dev/dm-0
        brw-rw---- 1 root root 253,  1 Jul 18 05:52 /dev/dm-1
        brw-rw---- 1 root root 253,  3 Jul 18 05:52 /dev/dm-3
        brw-rw---- 1 root root 253,  2 Jul 18 05:52 /dev/dm-2
        brw-rw---- 1 root root 253,  4 Jul 18 05:52 /dev/dm-4
        brw-rw---- 1 root root 253,  5 Jul 18 05:52 /dev/dm-5
        brw-rw---- 1 root root 253,  6 Jul 18 05:52 /dev/dm-6
        brw-rw---- 1 root root 253,  7 Jul 18 05:52 /dev/dm-7
        brw-rw---- 1 root root 253,  8 Jul 18 05:52 /dev/dm-8
        brw-rw---- 1 root root 253,  9 Jul 18 05:52 /dev/dm-9
        brw-rw---- 1 root root 253, 10 Jul 18 05:52 /dev/dm-10
        brw-rw---- 1 root root 253, 11 Jul 18 05:52 /dev/dm-11







        [root at cshao57 mapper]# ls -lrt
        total 0
        crw------- 1 root root  10, 63 Jul 18 05:52 control
        brw-rw---- 1 root disk 253,  0 Jul 18 05:52
        3600a0b80005b10ca00008e254c7726b1
        brw-rw---- 1 root disk 253,  1 Jul 18 05:52
        3600a0b80005b0acc00008c204c7744ba
        brw-rw---- 1 root disk 253,  3 Jul 18 05:52
        3600a0b80005b0acc00008c204c7744bap2
        brw-rw---- 1 root disk 253,  2 Jul 18 05:52
        3600a0b80005b0acc00008c204c7744bap1
        brw-rw---- 1 root disk 253,  4 Jul 18 05:52
        3600a0b80005b10ca00008e254c7726b1p1
        brw-rw---- 1 root disk 253,  5 Jul 18 05:52 HostVG-Swap
        brw-rw---- 1 root disk 253,  6 Jul 18 05:52 HostVG-RootBackup
        brw-rw---- 1 root disk 253,  7 Jul 18 05:52 HostVG-Root
        brw-rw---- 1 root disk 253,  9 Jul 18 05:52 HostVG-Logging
        brw-rw---- 1 root disk 253,  8 Jul 18 05:52 HostVG-Config
        brw-rw---- 1 root disk 253, 10 Jul 18 05:52 HostVG-Data
        brw-rw---- 1 root disk 253, 11 Jul 18 05:52 live-rw
        brw-rw---- 4 root root 253, 11 Jul 18 05:52 link2
        brw-rw---- 4 root root 253, 11 Jul 18 05:52 link1
        brw-rw---- 4 root root 253, 11 Jul 18 05:52 link
        [root at cshao57 mapper]# ls -lrt /dev/dm*
        brw-rw---- 1 root root 253,  0 Jul 18 05:52 /dev/dm-0
        brw-rw---- 1 root root 253,  1 Jul 18 05:52 /dev/dm-1
        brw-rw---- 1 root root 253,  3 Jul 18 05:52 /dev/dm-3
        brw-rw---- 1 root root 253,  2 Jul 18 05:52 /dev/dm-2
        brw-rw---- 1 root root 253,  4 Jul 18 05:52 /dev/dm-4
        brw-rw---- 1 root root 253,  5 Jul 18 05:52 /dev/dm-5
        brw-rw---- 1 root root 253,  6 Jul 18 05:52 /dev/dm-6
        brw-rw---- 1 root root 253,  7 Jul 18 05:52 /dev/dm-7
        brw-rw---- 1 root root 253,  8 Jul 18 05:52 /dev/dm-8
        brw-rw---- 1 root root 253,  9 Jul 18 05:52 /dev/dm-9
        brw-rw---- 1 root root 253, 10 Jul 18 05:52 /dev/dm-10
        brw-rw---- 4 root root 253, 11 Jul 18 05:52 /dev/dm-11


On Wed, Jul 20, 2011 at 04:34:38AM -0400, Chao Yang wrote:
> Hi there, 
> 
> I have questions about /dev/dm-* in my mind for days, it seems here is the right place for the answers. 
> What is a /dev/dm-* file? it says "/dev/dm-0: block special" from file command. 
> Is there a limitation to the number of /dev/dm-* files ? I tried with RHEL6.1 GA, created 800 from dm-0 to dm-799 as maximum. 
> Thanks. 
> 
> 
> Best, 
> Chao 
> 

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