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Re: [linux-lvm] /dev/dm-* or /dev/mapper/*



On 2009-08-16 19:08, Edward Siefker wrote:
I originally set up raid-luks-lvm on this machine with debian's
installer tool, now I am trying to add another encrypted raid1 device to
the existing volume group.  I already have the raid device set up and
encrypted, that's no problem.  Now the linux gazette
(http://linuxgazette.net/140/pfeiffer.html) tells me I should run
'pvcreate' on the device in /dev/mapper.  However, if I examine my
existing physical volumes with 'pvscan' I get the following:

iblis:/home/hatta# pvscan
  PV /dev/dm-0   VG iblis-volumes   lvm2 [931.32 GB / 0    free]
  Total: 1 [931.32 GB] / in use: 1 [931.32 GB] / in no VG: 0 [0   ]

Apparently I am using /dev/dm-0 instead of /dev/mapper/md1_crypt. I
wondered if these were maybe two names for the same thing, so I checked
ls:

iblis:/home/hatta# ls -ld /dev/dm-0 /dev/mapper/md1_crypt
brw-rw---- 1 root disk 253, 0 2009-08-16 12:02 /dev/dm-0
brw-rw---- 1 root disk 253, 0 2009-08-16 12:02 /dev/mapper/md1_crypt

Same major and minor number, if that means anything.  Next I ran
'dmcrypt info' on each:

iblis:/home/hatta# dmsetup info /dev/dm-0
Device /dev/dm-0 not found
Command failed
iblis:/home/hatta# dmsetup info /dev/mapper/md1_crypt Name: md1_crypt
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        7
Event number:      0
Major, minor:      253, 0
Number of targets: 1


It works on one, and not the other.  So they're not the same thing.  My
new device 'md2_crypt' corresponds to /dev/dm-8, if I am to trust the
major/minor numbers. Should I run pvcreate on /dev/dm-8 or
/dev/mapper/md2_crypt?

/dev/dm-* device names seem to go in sequence of when they are discovered at boot time, or order in the config file, or some other such algorithm who's input changes as LVs are added and destroyed.

You know, though, that md1_crypt will always be md1_crypt no matter what you do to any other LVs.

I'd run interactive commands against dm-8, but would refer to md2_crypt in any scripts that I write which need to work tomorrow, next week, etc.

--
Scooty Puff, Sr
The Doom-Bringer


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