[Fedora-xen] pygrub failed

[Winston Chang]

>> - It creates a whole disk image instead of importing partitions.
>> This makes it difficult to access domU filesystems from dom0, but
>> this is often necessary for management.
>
> That's just part of the goal of making the domU environment as similar
> as possible to the dom0.  Having a single disk image passed in  
> means you
> can keep boot info, root fs and swap all in the same place in the host
> OS.

I understand the motivation here, but it does make it much harder to  
manipulate the domU filesystem from dom0.  I personally couldn't  
figure out how to mount a domU partition when using the whole-disk  
method.  I imagine that it's even more complicated when the domU has  
its own LVM volumes.

This issue aside, I do like the idea of having a single disk image  
for each domU, though, since it keeps things simple.  It's nice to  
have the kernel, swap partition, and main partition (and whatever  
else) all in one place.  But it seems very inelegant to use the  
ancient DOS partitioning scheme and then put LVM on top of it.

Things would be simpler if a domU could start out with LVM right on  
top of the disk image, like if you ran 'pvcreate /dev/sda' in the  
domU.  Then you wouldn't need a separate /boot partition; it could be  
just another LVM volume.  I know that this is problematic for "real"  
installations of Linux, because of the BIOS <1024 cylinder limitation  
(is that still an issue?), and because grub doesn't understand LVM.   
But the first limitation simply doesn't exist with a domU under Xen  
(I hope), and the second can be worked around -- I imagine that  
making pygrub understand LVM would be easier than making grub  
understand LVM, since there aren't the same memory/space limitations.

In short, why emulate all crufty old mechanics of booting and  
partitioning a PC when Xen provides a way to (finally) avoid it?  You  
would lose the ability to image a domU disk image right onto a real  
disk and then boot a real machine with that disk, but I think that's  
the only thing, and it's pretty minor.




>> - It doesn't provide an option for creating images on LVM volumes.
>
> If you already have an LVM volume, you can simply specify that and it
> just works (most of my test xen installs are to LVM.)

That's good to know... The script doesn't make it clear that it has  
that capability.  If I remember right, I tried that, and the config  
file said file:/dev/VolGroup00/fc5_root, or something like that  
instead of phy:.., which seems like an error.  (I was still having  
other Xen problems so I never got it working this way.)  The LVM  
volume already has a size, so what happens when you specify a disk  
image size to the installer script?


>> Or you could use a kernel from xensource that has the xennet driver
>> compiled into the kernel.  I don't know why the developers decided to
>> take the driver out of the kernel.
>
> The default networking with Xen uses netfront/back and bridging, so
> that's what we compile in.  Is there really a strong need for xennet
> too?

I'm not an expert on Xen network drivers... All I know is that when I  
booted the jailtime.org FC5 image (and a Centos 4.2 image that I  
created myself) using the stock FC5 xenU kernel, that it would say  
there was no eth0 on boot.  I managed to get eth0 to show up by  
loading the xennet kernel module, as recommended by someone on the  
xensource xen-users list.  If there's way to get a domU's eth0 to  
show up using the stock XenU kernel and without loading a module,  
that would be great.  How would I do this?

--Winston