Re: [linux-lvm] pvcreate to memory

["Bryn M. Reeves" <bmr@redhat.com>]

On Sat, Sep 05, 2020 at 01:40:54AM +0200, Tomas Dalebjörk wrote:
> is there a way to create vgda information to memory?

The VGDA (volume group descriptor area) is a term from AIX's volume manager:
it's not going to apply directly to LVM2.

There are PV labels (headers), which identify a disk as belonging to LVM and
include the PV UUID, and there is the text metadata area (mda) which contains
the volume group metadata that describes an LVM2 VG and the logical volumes
that it includes. All physical volumes must have a PV label, but some PVs in
a VG (especially if it has many PVs) need not contain metadata.

If you understand this then you can push the limits of what you can do in
various ways and even construct devices "by hand" with dmsetup. This doesn't
mean you can do anything but the better you know the component parts the more
possibilities are available to you (and it's worth pointing out that these
configurations place a lot more responsibility on the user than normal - you
get to keep all the pieces when they break).

If you're interested in LVM internals you should start with something like
Alasdair's talk from a few years ago, and learn how the VG metadata read from
devices is translated into device-mapper tables.

It's easy enough to create a PV (and a volume group) on a RAM based device but
unless you understand how the various layers opperate you will have a very
hard time trying to get the tools to do what you want.

  http://people.redhat.com/agk/talks/LVM2-LinuxTag2006/

Regards,
Bryn.

> I want to create a virtual LVM disk
> We have all the cow data on a disk, but 
> # lvconvert -s vg/lv /dev/mydisk
> doesn’t accept the path to the disk device
> and if I create a symbolic disk in /dev/vg/mydisk of in /dev/mapper/vg—mydisk pointing to /dev/mydisk
> the ‘lvconvert’ command doesnt accept the cow data on the disk
> so I guess that the VGDA data has to be present on the disk in order for the lvconvert command to accept the data
> what do you think could be missing?
> is it the vgda meta data on the disk?
> 
> if so, how can I create and emulate the vgda data so that the lvconvert command works?
> 
> thanks in advance 
> regards Tomas
> 
> Sent from my iPhone
> 
> 
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