[linux-lvm] LVM2/DM Mapping Tables Documentation

bowmailtmp-lvm at yahoo.com bowmailtmp-lvm at yahoo.com
Tue Jul 13 23:38:43 UTC 2004

ok, i think i've got an idea what's going on here, it
seems that the lvm cache may be the problem here.  it
seems like in some cases if i wait a couple *minutes*
then the data magically shows up in the proper
physical sectors of the disk.  in some cases it is
faster, some quite slow.  can someone confirm this for
me and give me an idea of the mechanics involved with
the cache?  also, is there a command to force the
cache to be flushed out?  i'll keep looking through
the man pages for this.


--- bowmailtmp-lvm at yahoo.com wrote:
> 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
> machine)
> 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
> following:
> 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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> read the LVM HOW-TO at
> http://tldp.org/HOWTO/LVM-HOWTO/

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