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Re: [Xen-users] why LVM? why not Extended Partitions?

To: "Tomasz Chmielewski" <mangoo@xxxxxxxx>
Subject: Re: [Xen-users] why LVM? why not Extended Partitions?
From: "Emre Erenoglu" <erenoglu@xxxxxxxxx>
Date: Mon, 26 Nov 2007 16:47:34 +0100
Cc: Xen Users <xen-users@xxxxxxxxxxxxxxxxxxx>
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Well Thanks for the answers. Now it starts to make sense.

My problem seems to be related to the fact that my setup is a "home-setup", where I replace my VMWare Workstation with Xen. The problem occurs when you want to use your DomU sometimes as bare-metal and sometimes as a DomU, and if your domU kernel does not have necessary LVM modules and tools.

1) As domU bare-metal kernel can't see LVM volumes, I'm locked up at boot on bare-metal.
2) Even though DomU kernel has LVM, when you try to run it on bare-metal, you can't use LVM as a whole disk image, you can just use it like a partition. A disk image with partitions, inside a LVM volume, can't be easily mounted if I'm not wrong.

Another point maybe related to my setup, is that my LVM volume group is fully utilized and when I need to resize or shrink, I'm having a lot of trouble. I guess I need to read more about LVM.

Thanks a lot for the responses.



On Nov 26, 2007 4:38 PM, Tomasz Chmielewski <mangoo@xxxxxxxx> wrote:
Emre Erenoglu schrieb:
> Hi,
> This is for HVM guests.
> I'm using LVM at my home setup for my DomUs, but having some issues with
> "mounting, umounting" the partitions inside the DomUs, as they are
> actually a disk image. Some of the kernels of my DomU's don't have LVM
> support at all, making it another issue.
> Why don't we just use Extended Partitions? What is the added benefit of
> LVM to us? I read around that it also includes a little overhead. When
> it comes to extension, shringking, modification etc, extended partitions
> are just as good, as you can use many proved commercial products to
> resize your partitions easily.
> Am I missing something here?

Yes, in several points.

1. It's dom0 which needs LVM support; domUs don't need it. In fact,
whatever you use on dom0 as a storage for domU (LVM, disk, partition,
file etc.) doesn't matter from the domU point of view - in the end, domU
sees "a disk" anyway.

2. How many partitions can you have with extended partitions? I think
it's 16 maximum. I also think there is a maximum size (1 or 8 TB, I
don't remember) old-style partitioning can handle.
But I guess it won't matter for smaller setups anyway.

Let's consider you have 3 partitions with different size, and some free
space after them:

[ A     ][ B   ][ C       ][ free     ]

You find that partition B is too small, and you want to resize it? Super
easy with LVM, and lots of effort when using old-style partitions.

Still LVM not fun for you? It has some additional goodies (like
snapshots, etc.).

Tomasz Chmielewski

Emre Erenoglu
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