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[linux-lvm] lvm + loopback



Hi all,

I have the following setup: pv (/dev/sda2) -> vg (name: vg) -> lv (name: test)
I would like to create a loopback device on /dev/sda2 (using offset) so the loopback will be exactly the test logical volume.
It is working with a testfile but somehow It is not working with a "real device".

So the steps are the following:
1., Clean the lv and create a pattern that I should find at the begin of the loopback device
# dd if=/dev/zero of=/dev/mapper/vg-test
# echo 'AAAA' > /dev/mapper/vg-test
# sync
# hexdump -C -n 10 /dev/mapper/vg-test 
00000000  41 41 41 41 0a 00 00 00  00 00                    |AAAA......|
0000000a

2., Check the PE size (4M)
# pvdisplay |grep 'PE Size'
  PE Size               4.00 MiB

3., Check where is the first PE on the partition (1M)
# pvs -o +pe_start
  PV         VG   Fmt  Attr PSize   PFree 1st PE 
  /dev/sda2  vg   lvm2 a--  465.56g    0    1.00m

4., Check where the lv is located on the pv (starting at the 11776 PE)
# lvdisplay -m vg/test
  --- Logical volume ---
  LV Name                /dev/vg/test
  VG Name                vg
  LV UUID                vT5X5z-1CJH-uw4n-Jcs8-DW50-PDDB-N6PlIu
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                419.56 GiB
  Current LE             107408
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:4
   
  --- Segments ---
  Logical extent 0 to 107407:
    Type linear
    Physical volume /dev/sda2
    Physical extents 11776 to 119183

5., Create a loopback device on /dev/sda2 (pv) with offset based on the following calculation: Where is the first PE on the device (1M) + Where is the firt PE of the test lv (11776)*PE size (4M) = 47105M
# losetup -f -v -o 47105m /dev/sda2
Loop device is /dev/loop0

6., Test it with hexdump (it should give the same result like in the firts step but id doesn't do it)
# hexdump -C -n 10 /dev/loop0
00000000  00 00 00 00 00 00 00 00  00 00                    |..........|
0000000a



Could somebody give me a hint where is the problem?


Thanks and regards,
B



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