Re: [Xen-users] Re: Partition vs disk images

["Fajar A. Nugraha" <fajar@xxxxxxxxx>]

Rob Greene wrote:
> > On Tue, Apr 29, 2008 at 10:15 PM, Fajar A. Nugraha <fajar@xxxxxxxxx> wrote:
> >
> > In RHEL 5 at least, anaconda requires the block device/file to be mapped as 
> > disk (e.g hda, sda) not partition (e.g. sda1). Try changing it to
> > disk = [ "phy:lvm-raid/FileVolGroup,sda,w" ]
> >
> >     
>  Ok, so (at least in CentOS/RHEL?) the Xen domU sees the dom0
> partition not as a partition but as a physical disk?  
Mark's post put this in a well-phrased statement : "the *installer* used 
by CentOS/RHEL
guests will be unhappy if you try and provide it with separate partitions
instead of a whole disk"

It's redhat installer issue, not a Xen issue. In other words, if you 
want to use redhat installer to setup domU, you should map it as a disk, 
not partition.
> sda1, sda, and
> hda all failed
Try xvda. That's the default device name used when you use virt-manager.

> What concerns me is that if/when I want to grow disk available to a
> domU (this on in particular is a file server), I can't just grow what
> shows up as hda1, right?  I'd need to add a new "disk" and extend a
> LVM within the domU?
>   
Yup.

> Don't these layers of RAID+LVM (dom0) and Xen block device and LVM
> (domU) come at a price?  
Around 3% performance penalty, I believe.

> I was hoping that the partition got mapped
> straight into the domU so I avoided any extra stuff within the domU
> and I would have the added bonus of being able to mount the drives and
> copy files between when setting this up.
>
>   
The "recommended" way, if you want to use virt-manager and redhat 
installer, is to have LVM on domU side only. Meaning :
- you map partitions or disks (not LVM) on dom0 as whole disk on domU
- the installer will setup LVM (in domU) on that disk
- to extend domU's fs, map another partition/disk to domU as whole disk, 
and use LVM on domU to extend the VG and LV

> > Having said that, if you're building lots of identical domUs, using prebuilt 
> > template (and mappingthe block device  as sda1 instead of sda) should be faster 
> > than installer.
> >     
> If I can only do partitions, how does this work then?  Unless all the
> drives are identical in size and I 'dd' the device??
>   
Not necessarily :) You can always use tar to copy the files.

I have prebuilt images of RHEL4 and RHEL5 domUs. To create a domU, what 
I did was :
- create two LVMs on domU : e.g. testrootlv and testswaplv
- initialize the LVMs : mkfs and mkswap
- create Xen config file, which is something like

==========================
memory = "250"
disk = [ 'phy:/dev/vg/testrootlv,hda1,w','phy:/dev/vg/testswaplv,hda2,w' ]
vif = [ 'mac=00:16:3E:23:3A:51, bridge=br105', ]
bootloader="/usr/bin/pygrub"
==========================

Note that in "vif" line, I specify MAC address and bridge (I setup 
network bridges manually, but that's another story). You should always 
define a static, unique MAC address for each domU for production 
purposes. The partition names can be or xvda1 or xvda2 if you want.
- populate root fs :
 mount /dev/vg/testrootlv && (wget -O - 
http://location_of_install_images | tar xvfz -) && umount /mnt/tmp
- startup domU. In my setup in always start with eth0 disabled, empty 
root password
- edit /etc/sysconfig/network and 
/etc/sysconfig/network-scripts/ifcfg-eth0, set root password
- reboot domU

The entire process took about five minutes.

You can convert an installed domU (created using installer) to a 
template quite easily, but I wont cover it here. Kai's post shows one 
way to do it.
Regards,

Fajar

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