[lvm-devel] LVM2 label and metadata location

[David Teigland]

On Sat, Oct 13, 2018 at 10:44:22PM +0200, Schneider, Janine wrote:
> I did read the LVM2 documentation and i have tried to find the location
> parsing in the LVM2 code but unfortunately I was unable to figure out how
> this has to be done.

This brief summary might get you started; the structure of the lvm code
doesn't make it easy to find or follow.

The second sector of the disk (offset 512) contains: struct label_header
followed by struct pv_header.  The pv_header (disk_areas) point to the
location of one (or optionally two) struct mda_header.  The first
mda_header is at offset 4096.  (The optional second one is at the end of
the device.)

The sector containing mda_header is followed by many sectors (e.g. 1MB).
These sectors act as a circular buffer containing lvm text metadata.  One
copy of the text metadata is written into this buffer after the last
version.  The mda_header's raw_locn struct points to the latest copy of
the metadata in this buffer.

_find_lvm_header() reads label_header.
_text_read() reads pv_header.
_raw_read_mda_header() reads mda_header
text_read_metadata() reads text metadata