[linux-lvm] VGDA Question

[linux-lvm] VGDA Question

[Rik Herrin]

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?  Shouldn't it just
have PE descriptors and leave the LV descriptors to
the volume group?  Also, when booting, what makes the
kernel know that there are VGs and LVs?  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?  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?. 
Finally, where exactly is the VGDA information held?. 
Thanks for your time.


	
		
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Re: [linux-lvm] VGDA Question

[Luca Berra]

On Wed, Sep 07, 2005 at 07:46:00AM -0700, 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?  Shouldn't it just
>have PE descriptors and leave the LV descriptors to
i tried, but fail to understand your question.
The VGDA is the only place on disk where information necessary for
activation of the volume group and contained logical volume can be
stored permanetly.

>the volume group?  Also, when booting, what makes the
>kernel know that there are VGs and LVs?  Does it look
The kernel doesn't. lvm is setup by user space tools.

>Finally, where exactly is the VGDA information held?. 
>Thanks for your time.
at the beginning of the PV
regatds

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Luca Berra -- bluca@comedia.it
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Re: [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

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