[linux-lvm] LVM2/DM Mapping Tables Documentation

[Dave B]

This is still causing me headaches... Does anybody
have any insight on this?

--- bowmailtmp-lvm at yahoo.com wrote:
> 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.
> 
> thanks,
> dave
> 
> --- 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.
> > 
> > 
> > 
> > _______________________________________________
> > linux-lvm mailing list
> > linux-lvm at redhat.com
> > https://www.redhat.com/mailman/listinfo/linux-lvm
> > read the LVM HOW-TO at
> > http://tldp.org/HOWTO/LVM-HOWTO/
> > 
> 
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> http://tldp.org/HOWTO/LVM-HOWTO/
>