[linux-lvm] loop devices?

[Mika Lansirinne]

Hi all,

I was trying to use a loop device(s) for LVM. In the pvcreate man-page it is 
said that a PV can also be a loopback file. That also seem to work correct. 
But when I try to create a VG on that I get:

[root at arthur /tmp]# vgcreate tmpvg /dev/loop0
vgcreate -- no valid physical volumes in command line

Pvdisplay gives:

[root at arthur lib]# pvdisplay /dev/loop0
pvdisplay -- ERROR "pv_read(): pv_create_name_from_kdev_t" no VALID physical 
volume "/dev/loop0"

'pvdisplay -d /dev/loop0' gives in the end:
...
<22> pv_create_name_from_kdev_t -- CALLED with 7:0
<333> lvm_check_dev -- CALLED
<4444> lvm_check_partitioned_dev -- CALLED
<55555> lvm_get_device_type called
<55555> lvm_get_device_type leaving with 3
<4444> lvm_check_partitioned_dev -- LEAVING with ret: TRUE
<333> lvm_check_dev -- LEAVING with ret: 1
<333> lvm_dir_cache -- CALLED
<333> lvm_dir_cache -- LEAVING with ret: 22
<22> pv_create_name_from_kdev_t -- LEAVING with dev_name: (null)
<1> pv_read -- LEAVING with ret: -278
<1> lvm_error -- CALLED with: -278
<1> lvm_error -- LEAVING with: "pv_read(): pv_create_name_from_kdev_t"
pvdisplay -- ERROR "pv_read(): pv_create_name_from_kdev_t" no VALID physical 
volume "/dev/loop0"

So it looks like that pv_create_name_from_kdev_t has the correct input for 
loop0, but lvm_dir_cache does not know anything about loop devices. (When I 
use disc partion for a PV, pv_create_name_from_kdev_t  leaves with a 
correct device file name).


BTW, I'm using RH 7.2 and 2.4.16 kernel. I tried this on the two different 
machines with the same results.

Has loop devices ever worked and has anybody actually used them?


For me being uninitiated it is a bit hard to understand why would lvm have to 
guess (and fail) a device file name from its major and minor numbers in the 
first place when I just give that in the command line...


-Mika

P.S. Thank you for the good work otherwise

-- 
Mika Länsirinne, System Manager / Research Engineer, M.Sc. (E.E.)
Helsinki University of Technology
Electronic Circuit Design Laboratory
P.O. Box 3000				Email: mla at ecdl.hut.fi
FIN-02015 HUT				Tel: +358 9 451 2277
Finland					Fax: +358 9 451 2269