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Re: [Xen-users] SAN / LVM backend and partitions

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] SAN / LVM backend and partitions
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Wed, 24 Jan 2007 18:58:19 +0000
Cc: Reinhard Brandstaedter <Reinhard.Brandstaedter@xxxxxx>
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> I'm playing around with Xen using LVM2 based on a SAN storage for VBDs.
> Unfortunately I'm not very familiar with LVM2 but so far I get DomUs up
> and running with using a logical volume as disk.

Well, that's good progress.  I'm not very familiar with LVM either ;-)

> >From the XEN User Manual I see this procedure:
>
> - create logical volume
> - create filesystem on LV
> - copy system files (the LV is mountable in dom0)
>
> Basically if you want multiple disks (or partitions for /usr,/var, ...)
> in the DomU you specify them in the vm config file and need to have
> multiple logical volumes on the Dom0 for that, like:
> /dev/vg0/vm-root
> /dev/vg0/vm-var
> /dev/vg0/vm-usr

This is handy because it makes them appear in a nice separate way in dom0, 
convenient to mount, extend, etc, etc.

It won't look quite like a "normal" block device to guest users though, and 
they won't be able to adjust the partitioning themselves.  Also, it won't 
work nicely in HVM guests - meaning that if you ever want to deploy HVM 
guests (or convert existing ones to HVM) you'll have to convert to the other 
approach.

> However I can also only specify only one logical volume in the config
> file and do the partitioning in the DomU then. (e.g. if I use
> virt-install on RedHat). So the one logical volume would get a partition
> table?
> Calling fdisk on this volume would print something like that:
> # fdisk -l /dev/vg0/vm-root
>
> Disk /dev/vg/vm-root: 6442 MB, 6442450944 bytes
> 255 heads, 63 sectors/track, 783 cylinders
> Units = cylinders of 16065 * 512 = 8225280 bytes
>
> Device             Boot  Start  End  Blocks    Id  System
> /dev/vg0/vm-root1  *     1      13   104391    83  Linux
> /dev/vg0/vm-root2        14     78   522112+   82  Linux swap
> /dev/vg0/vm-root3        79     783  5662912+  83  Linux
>
> Maybe this is much more a LVM issue, but which way is the better one for
> use with Xen?
> If I use partitions in a logical volume can they be mounted in the Dom0
> system as well? (e.g. for changing files while DomU is not running)
> Are there any considerations for these two aproaches when using
> live-migration?

You can use the lomount tool (or others) to mount partions within the LV from 
dom0 (again, whilst the domU isn't running).  It's not *quite* as convenient 
to work with from dom0's PoV but it looks more "normal" to the guests.  I've 
moved over to this configuration now, primarily because it makes it easy to 
convert to / from fully virtualised guests, and to perform initial installs 
in Qemu.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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