[linux-lvm] Root on LVM and vgscan

[Heinz J. Mauelshagen]

On Mon, May 28, 2001 at 01:44:34PM +0100, Dick Middleton wrote:
> 
> Thanks for your answers - as usual they provoke more questions...
> 
> >  On Fri, May 25, 2001 at 09:28:34AM +0100, Dick Middleton wrote:
> >  > I have root running on lvm on sw raid partitions on linux 2.2.18.  It all
> >  > works fine.  However I have had difficulty during booting after adding new
> >  > LVs.  The symptom is that the initial mounts of LVs fails or the wrong
> >  > partition is mounted.
> >  > 
> >  > What happens is that vgscan is run during the initrd phase and so the lvmtab
> >  > is updated on the ram disk.  Once the root file system on the hard disk has
> >  > been mounted the lvmtab on that comes into use.  That has not been updated by
> >  > vgscan and so does not reflect the changes. 
> >  > 
> >  > Obviously it's possible to run vgscan again (between remounting root rw and
> >  > checking the other discs) to update the lvmtab but it does beg a few
> >  > questions.
> 
> >  Yes, that's the option (forcing some overhead).
> >  The other would be to get the actual /etc/lvmtab* from the initial ram disk :-)
> 
> Ah yes, I discovered that it lives in /initrd! Can I assume from this that
> it's safe to copy the lvmtab.  Is that a good solution?

Sure.
This has the actual vgscan results *and* causes as little overhead as possible
to get it into the real root.

> 
> >  > 
> >  > 1) why does mount use the data in lvmtab and not in /proc to find out about
> >  >    active VGs?
> 
> >  mount doesn't access /etc/lvmtab* at all.
> 
> Now you've got me.  Why/how does mount mount the wrong partition when lvmtab
> is out of date if it doesn't access lvmtab? Are /dev/<vg>/<lv> updated by
> vgscan as well? It looked to me as if they weren't. Do they need to be copied
> from /initrd too?

Actually I've been too brief before :-(

The device nodes in your real root are the ones created *before* vgscan
was called from linuxrc and are likely out of date WRT to the minor numbers
they contain.


Something like

/dev/VGName/LVname 58 1

could be in your real root *but*

/dev/VGName/LVname 58 2

could have been updated by the vgscan run in the initial ram disk fs.

IOW: you can copy the /etc/lvmtab* files from the initial ram disk *and*
     run vgmknodes(8) from your real root based startup script *before* the
     VG is accessed (eg. to mount a fs).

> 
> What should I read to better understand how all this works?
> 
> Dick
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-- 

Regards,
Heinz    -- The LVM Guy --

*** Software bugs are stupid.
    Nevertheless it needs not so stupid people to solve them ***

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Heinz Mauelshagen                                 Sistina Software Inc.
Senior Consultant/Developer                       Am Sonnenhang 11
                                                  56242 Marienrachdorf
                                                  Germany
Mauelshagen at Sistina.com                           +49 2626 141200
                                                       FAX 924446
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