[Xen-users] hvm questions (hvm vs. patched DomU kernel)

[Christian Anton <mail@xxxxxxxxxxxxxxxxx>]

Hi folks,

i have been doing some tests with xen in the last weeks. First only with
patched Kernels in the DomUs, now using the hvmloader, too.

I think using the hardware capabilities of the new Intel Dualcore CPUs is
not yet very well documented, so i want to ask some questions here.

1. hvmloader vs. vmxloader
In the official documentation of xen 3.0 they talk about using the
vmxloader to use unmodified DomU kernels. Is this an error in the
documentation?
http://www.cl.cam.ac.uk/Research/SRG/netos/xen/readmes/user/user.html#SECTION04300000000000000000
I am using hvmloader for my tests as i think that vmxloader belongs to the
2.x version and has been replaced by hvmloader in version 3.

2. Handling of unmodified guests
I enjoyed the handling of patched guest os's a much, because the xm
console (the virtual serial console) gave me much comfort setting up
networking and services in a DomU.

Is it possible to use this on unmodified guests too? Do i have to
configure a serial port in the virtual machine and do some configuration
to create a virtual serial port to connect to with xm console / minicom?
Is this documented anywhere?

Is it possible to start a virtual machine with a fixed ID? In the config
file i give the machine a name, but when i reboot it, it gets a new
integer id what means, that the vnc-server for this machine is listening
on another port, and as my test box is only reachable via ssh, i must do a
new ssh connect with another port forwarding for every time i reboot a
virtual machine.

3. Disks (LVM, Files...)
I use LVM logical volumes for my DomUs with patched kernels passing them
to the domUs as partitions, such as hda1, hda2... The big advantage of
this is that i can copy a virtual machine with rsync, then configure the
nessecary things using a chroot, leave and power up the machine. Great!
Doing backups using LVM snapshots would be something cool, too. Using
the hvmloader it seems that this is not possible at all, i can only pass
LVM volumes or files as entire disk (hda, hdb) and the guest operating
system must partition it. So i am not able to access the files on the
virtual disk when the machine is not running, right? Is there any chance
to use a similar setup as i use for modified guests?

Also i have noticed that i get a disk IO performance of 57Mbyte/sec
buffered disk reads with hdparm -tT in a patched domU, nearly the value i
get in Dom0, but in a unmodified DomU i get poor results, just about
9Mbyte/sec. What is the reason of this?

4. Recommendation
Is it recommendet to use xen with unmodified DomUs when using Linux as
virtual machines operating system? I thought it would be faster than
patched kernels because the CPU is doing the hard job instead of the
kernel patches, but for now it does not seem so. I did not yet do
CPU performance tests, so the only thing i have noticed until now is the
poor disk io.

5. qemu
In my tests i have noticed that when i am using hvmloader the
virtualization job is done by a process named "qemu", is this the qemu i
know? Was it adapted to work with xen or is it a xen development that has
accidently the same name as http://freshmeat.net/projects/qemu/?

It would be really great if you could answer my questions. I am wanting to
use xen instead of our vmware gsx-server we have now, virtualizing some
Windows but mostly Linux servers.

In vmware we have io issues with our servers, and the vmware environment
produces high load on the host system "without doing anything".


Greets

Christian Anton

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