What is a /dev/dm-* file ?

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* What is a /dev/dm-* file ?
       [not found] <1353628002.1450009.1311150616312.JavaMail.root@zmail01.collab.prod.int.phx2.redhat.com>
@ 2011-07-20  8:34 ` Chao Yang
  2011-07-20 10:17   ` li mohua
  0 siblings, 1 reply; 3+ messages in thread
From: Chao Yang @ 2011-07-20  8:34 UTC (permalink / raw)
  To: dm-devel

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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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^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: What is a /dev/dm-* file ?
  2011-07-20  8:34 ` What is a /dev/dm-* file ? Chao Yang
@ 2011-07-20 10:17   ` li mohua
  2011-07-21  9:35     ` Bryn M. Reeves
  0 siblings, 1 reply; 3+ messages in thread
From: li mohua @ 2011-07-20 10:17 UTC (permalink / raw)
  To: device-mapper development


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@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@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@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@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 
> 

> --
> dm-devel mailing list
> dm-devel@redhat.com
> https://www.redhat.com/mailman/listinfo/dm-devel


-- 
我只担心一件事，我怕我配不上我所遭受的苦难

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^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: What is a /dev/dm-* file ?
  2011-07-20 10:17   ` li mohua
@ 2011-07-21  9:35     ` Bryn M. Reeves
  0 siblings, 0 replies; 3+ messages in thread
From: Bryn M. Reeves @ 2011-07-21  9:35 UTC (permalink / raw)
  To: device-mapper development; +Cc: li mohua

On 07/20/2011 11:17 AM, li mohua wrote:
> 
> 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, 

On a modern system it's the other way around - the /dev/mapper entries are
symbolic links to ../dm-N entries maintained by a set of udev rules (look at
/lib/udev/rules.d/10-dm.rules on a RHEL6 system).

The /dev/$VG/$LV directories and symlinks for LVM2 volume groups and logical
volumes are maintained by a similar set of rules in 11-dm-lvm.rules.

Device mapper minor numbers are assigned dynamically so the upper limit is
dependent on the system (amount of memory) and the types of devices that you are
trying to create (number and type of targets used) - it's not that unusual to
see systems with 1000s of dm devices.

>> 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. 

Generally it's advised not to use the dm-N device nodes directly since they do
not provide persistent naming and are created as soon as a device-mapper device
is assigned a minor number in the kernel but possibly before the device is in a
usable state.

Depending on the device-mapper application you're using (LVM2, multipath-tools,
dmraid etc.) there are probably better ways to reference the devices via the
types of symlinks mentioned above.

Regards,
Bryn.

^ permalink raw reply	[flat|nested] 3+ messages in thread

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     [not found] <1353628002.1450009.1311150616312.JavaMail.root@zmail01.collab.prod.int.phx2.redhat.com>
2011-07-20  8:34 ` What is a /dev/dm-* file ? Chao Yang
2011-07-20 10:17   ` li mohua
2011-07-21  9:35     ` Bryn M. Reeves

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