[linux-lvm] dmsetup -split is buggy

[linux-lvm] dmsetup -split is buggy

[Linda A. Walsh]

I'm told that /dev/VG/LV is the official LVM syntax for a devname,

so I passed a /dev/VG/LV, name to LVM, and it mangled the LV name,
splitting it into a LVLayer.




I gave it:

/dev/Home+Space/Home-2011.08.08-03.29.58


Which corresponds to a /home/snapdir/@GMT-2011.08.08-03.29.58
(@GMT-date-time is a required name format), so my volumes
for such snapdirs have the base-LV and the date-time.


There is nothing in the lvm spec that or device name spec, that says
"-" is to be interpreted as some layer.... 

The problem is that when this routine gets a device name, it might be
/dev/mapper/<encoded value>
or /dev/VG/LV

dmsetup splitname, should, I believe, know how to split
names in the only 'supported' name format  i.e. /dev/VG/LV -- but
instead, it's treating it as though it were under /dev/mapper and using 
that for
decoding...

I can hack around the bug, but its surprising that dmsetup doesn't support
the only official LVM names (/dev/VG/LV).

Is this bug a 'feature', or is it something that should be fixed?

Re: [linux-lvm] dmsetup -split is buggy

[Stephane Chazelas]

2011-08-15 02:34:20 -0700, Linda A. Walsh:
[...]
> I can hack around the bug, but its surprising that dmsetup doesn't support
> the only official LVM names (/dev/VG/LV).
> 
> Is this bug a 'feature', or is it something that should be fixed?
[...]

dmsetup deals with device-mapper devices. Those in /dev/mapper,
that you can see with "dmsetup table". LVM uses the
device-mapper, and is one of the many subsystems that use it.
You can even create device mapper devices manually and use
whatever convention you like like.

sudo dmsetup table VG-LV | dmsetup create LV@VG

for instance, to make a clone of the VG-LV device (created by LVM from
the VG/LV subvolume) as a LV@VG device.

If there's a bug in dmsetup, it's with providing with a
"splitname" subcommand. I can't think of any good reason why
that is included. IMO, that feature should be provided by LVM,
not dmsetup.

-- 
Stephane

Re: [linux-lvm] dmsetup -split is buggy

[Peter Rajnoha]

On 08/15/2011 11:34 AM +0100, Linda A. Walsh wrote:
> dmsetup splitname, should, I believe, know how to split
> names in the only 'supported' name format  i.e. /dev/VG/LV -- but
> instead, it's treating it as though it were under /dev/mapper and using that for
> decoding...

dmsetup splitname accepts a device-mapper name as an argument,
/dev/VG/LV is a "logical volume path" and is not supposed to be
decoded this way...

Peter