[linux-lvm] LVM2/DM Mapping Tables Documentation

bowmailtmp-lvm at yahoo.com bowmailtmp-lvm at yahoo.com
Tue Jul 13 21:23:12 UTC 2004

I have been having a very difficult time correlating
physical addresses to LVM2/DM addresses.

for instance, if i do:
dmsetup table /dev/mapper/avtest-vdisk2g

the result is:
0 4194304 linear 22:65 384

from what i currently understand, this means logical
extent 0 (which is composed of 8192 512 byte sectors
in the default config) of this LV should start at
physical sector 384 of device 22:65 (/dev/hdd1 on my

after i've written to the logical device a couple
times, it appears this correlation doesn't hold when i
access the physical sectors on hdd1 using dd. 
however, if i use dd to access the sectors through the
LV, all the data appears as it should.  i was under
the impression that the logical to physical mappings
was quite straightforward and intuitive, but i feel
that i am missing some key information.  Can someone
help me out of is there decent developer documentation
somewhere that explains the mappings?

As a more complete example of my problem, if i do the

dd if=/dev/zero of=/dev/avtest/vdisk2g count=1
then do:
dd if=/dev/avtest/vdisk2g of=tmpfile.out count=1
then tmpfile.out contains 512 bytes of zeros.

however, if I do:
dd if=/dev/hdd1 of=tmpfile.out skip=384 count=1
then tmpfile.out contains other data, not the zero's i
would expect from the output of the dmsetup table
command above.  I feel i'm missing something fairly
basic here, but I haven't had any luck tracking this
relationship down.  I suppose there is always the
sourc code :)

Thanks a lot.

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