Re: [linux-lvm] LVM2/DM Mapping Tables Documentation

[Dave B <bowmailtmp-lvm yahoo com>]

Thanks for your reply!

yeah, i've tried this  and it does do the trick for
physical data queued up in the block device interface,
but since LVM/DM provides an extra level of
abstraction running sync at prompt, or even embedding
fsync() into the C code after writes apparently has
little to no effect on LVM volumes.
I actually waited a couple HOURS the other day for
data on a LV to actually show up in the promised
sectors of the physical disk.

-dave


--- Angelo DiNardi <adinardi slshs org> wrote:
> I believe the command "sync" at a shell prompt will
> flush the cache to 
> disk. Someone correct me if I'm wrong.
> 
> --Angelo
> 
> Dave B wrote:
> > This is still causing me headaches... Does anybody
> > have any insight on this?
> > 
> > --- bowmailtmp-lvm 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 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 redhat com
> >>>https://www.redhat.com/mailman/listinfo/linux-lvm
> >>>read the LVM HOW-TO at
> >>>http://tldp.org/HOWTO/LVM-HOWTO/
> >>>
> >>
> >>_______________________________________________
> >>linux-lvm mailing list
> >>linux-lvm redhat com
> >>https://www.redhat.com/mailman/listinfo/linux-lvm
> >>read the LVM HOW-TO at
> >>http://tldp.org/HOWTO/LVM-HOWTO/
> >>
> > 
> > 
> > _______________________________________________
> > linux-lvm mailing list
> > linux-lvm 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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