[linux-lvm] VGDA Question

[Markus Baertschi]

Rik Herrin wrote:

>Hi,
> According to the LVM man page, the VGDA (Volume Group
>Descriptor Area) holds the metadata responsible for
>keeping information about the LVs.  However, it is
>contained in the PVs.  Why does the VGDA contain both
>LV descriptors and PE descriptors?
>
Because there are LV's with PE's and both types of information must be 
stores somewhere.

>  Shouldn't it just
>have PE descriptors and leave the LV descriptors to
>the volume group ?
>
The VGDA is where the volume group information is stored. The LV info is 
therefore exactly where it belongs.

>  Also, when booting, what makes the
>kernel know that there are VGs and LVs?
>
The vgscan command is run during boot (from a rc file) and it detects 
the volumegroups and makes them available.

>  Does it look
>at the type of the partition and if it detects that
>it's a PV, extract the VGDA to help it build this
>information?
>
Yes

>  If this is so, how does it deal with
>entire harddisks.  For example, if I run the command:
>pvcreate /dev/sdd
>how would the kernel know that this is a PV?. 
>  
>
If you use the enire harddisk, then there is no traditional partition 
table. In this case the vgscan command looks if the entire disk is a PV.

>Finally, where exactly is the VGDA information held?.
>
At the beginning of the disk/partition you created with pvcreate.

Markus

-- 
  Markus Baertschi             Phone: ++41 (21) 807 1677
  Bas du Rossé 14b             Fax  : ++41 (21) 807 1678
  CH-1163, Etoy                Email: markus at markus.org
  Switzerland                  Homepage: www.markus.org