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Re: [Xen-users] Re: Cannot boot with LVM inside Xen DomU

To: Oliver Wilcock <oliver@xxxxxxx>, "Fajar A. Nugraha" <list@xxxxxxxxx>
Subject: Re: [Xen-users] Re: Cannot boot with LVM inside Xen DomU
From: Prakhar Srivastava <prakhar.apj@xxxxxxxxx>
Date: Tue, 12 Jul 2011 00:17:57 +0530
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Thanks a lot to both of you for providing all these valuable information. This has really enhanced my understanding about LVM. The idea  of copying the entire physical partition to a logical volume was really bad.

So, I tried to use virt-install to create a Xen guest image. However I found that virt-install was easier to use with Red Hat/CentOS because it doesnt work out of the box for debian/ubuntu. The installer itself partitioned the Xen virtual disk into root and swap LVs. .I had some issue with CentOS installer where the installer hanged at 0% while downloading essential packages. But I believe I will be able to sort it out. 

I have one more query. As stated by Fazar in his last reply, Redhat/CentOS DomU have by default LVM within which allows them to increase the root disk online by xm block-attach command. However, I have read in one of the Xen list discussion that using LVM as DomU backend and also within DomU is not a great idea in terms of performance. I want to have a similar kind of setup and believe that having the flexibility of resizing root disk online is a great plus in a virtualized environment. What are your thoughts on this ??

On Mon, Jul 11, 2011 at 10:51 PM, Oliver Wilcock <oliver@xxxxxxx> wrote:
I wanted to echo Fajar's comments that using the installer to create a
guest with a new LVM configuration inside the virtual disk is the best
idea for you.

The idea that you could copy /dev/sda into a LV is rather suspicious.
Only an LV created on a larger device such as /dev/sdb could contain the
data in /dev/sda.  If you created the LV in a VG created on /dev/sda then
it would not fit inside itself.

Also, please understand that an LVM configuration has named volume groups
(VGs).  If you copy /dev/sda then the name of the VG which is stored in
/dev/sda would also be copied.  This means two VGs with the same name on
the same computer.  That's bad news.

It might not be apparent to you that VGs have names if you haven't created
many.  For example, the openSUSE installer defaults to a VG named system.
Each VM guest would try to create "system" and this is not such a great
idea.  I would change each to match the name of the server.

> Date: Sun, 10 Jul 2011 22:54:23 +0530
> From: Prakhar Srivastava <prakhar.apj@xxxxxxxxx>
> Subject: Re: [Xen-users] Cannot boot with LVM inside Xen DomU
> To: "Fajar A. Nugraha" <list@xxxxxxxxx>
> Cc: xen-users@xxxxxxxxxxxxxxxxxxx
> Message-ID:
>       <CAF+Dr+uesk7reRgujQkM52f9HybYEN5dADUAj07Li3nqr_ss5A@xxxxxxxxxxxxxx>
> Content-Type: text/plain; charset="iso-8859-1"
> Hi,
> I am going to copy the domO PV i.e. /dev/sda to a logical volume and use
> that to boot a Xen guest.
> I have been using Eucalyptus images for Xen to rum them from an LVM
> backend.
> Anyways, I am going to try and use virt-manager for installing Xen PV
> guests
> and hope I am able to setup LVM within.
> Thanks
> Prakhar

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