[linux-lvm] Identifying useable block devices

Oliver Rath rath at mglug.de
Wed Jan 15 16:17:51 UTC 2014

Hi Marius!

the /dev/mapper/.. devices are for internal use only. This is needed,
because the thinPOOL is a device logical between vg and lv, which is
represented with help of these special devices.

Normally you should alway use the /dev/vg/lv name scheme for you own
lvm-devices. Nevertheless, if you would persist to use /dev/mapper/
pathes, you can create the path with /dev/mapper/<vg>-<lv>. Note: If you
lv-name includes some "-", they will be doubled in /dev/mapper notation
for differentiating to the "-", which separates vg and lv name. I.e. you
LV is name my-own-lv and your VG is mariusvg, then you can find the
(thintpool-) device under /dev/mapper/mariusvg-my--own--lv and (better)
as link /dev/mariusvg/my-own-lv.

There is a point you have unfortunatly to use the /dev/mapper-notation:
If you need i.e. kpartx for creating subdevices (i.e. the lv includes a
virtual windows disk with partitions), then the temporalily created
names are only accessible via /dev/mapper.

The other created metadevices for thinpool-managment you should never touch.


On 15.01.2014 09:19, Marius Vollmer wrote:
> Hi,
> when looking at the udev properties of a device mapper node, how can I
> decide whether this is a block device that the user can use for creating
> filesystems on, etc?
> For example, when creating a thinly provisioned logical volume with these
> steps
>  # vgcreate TEST /dev/loop1
>  # lvcreate TEST --thinpool pool -L 80
>  # lvcreate -T TEST/pool -n thin -V 100
> I end up with a lot of devices:
>  # ls /dev/mapper/TEST-*
>  /dev/mapper/TEST-pool        /dev/mapper/TEST-pool_tmeta  /dev/mapper/TEST-thin
>  /dev/mapper/TEST-pool_tdata  /dev/mapper/TEST-pool-tpool
> How can a program tell that only /dev/mapper/TEST-thin can really be
> used as a block device, and the rest should be ignored?
> Is there a way to do this by looking at "udevadm info", for example?
> (What seems to work is skipping all nodes that have
> DM_UDEV_IGNORE_DISK_RULES_FLAG set to true.  Is this maybe even
> documented somewhere?)
> Thanks!
> _______________________________________________
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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

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