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Re: [Xen-users] What are the advantages of running domU in a lvm logical

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Subject: Re: [Xen-users] What are the advantages of running domU in a lvm logical volume?
From: Freddie Cash <fjwcash@xxxxxxxxx>
Date: Thu, 10 Apr 2008 09:42:51 -0700
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On April 10, 2008 06:47 am James Pifer wrote:
> I've built my second xen server using SLES10SP1. I learned quite a bit
> on the first server with help from this list. This second xen server is
> going to be used for hosting mostly windows domU's.
> I've followed the instructions here:
> http://ian.blenke.com/vmware/vmdk/xen/hvm/qemu/vmware_to_xen_hvm.html
> I've successfully converted two vmware images to xen domU's. It worked
> amazingly well!
> I have three questions.
> 1) The last part of instructions has you create a logical volume and dd
> the disk image to it. Based on my experience with the first server, I
> left a whole bunch of free space for doing this.
> But why do this? Why not just run the domU from the img file? I tested
> it on the ones I did and it seems to run fine. Just wondering if there
> are specific advantages for running from a logical volume.

Image files make things more portable (just copy the file around), but 
slow things down as you have a filesystem in a partition (domU) in a file 
on a filesystem (dom0) on a block device (phy).

LVM partitions make things less portable (you have to clone the partition 
instead of just copying a file) but speed things up as you just have a 
filesystem in a partition (domU) on a block device (phy).

Or something along those lines.

> 2) For a windows domU, if you restart dom0, is the domU shutdown
> gracefully? I clicked shutdown from virt-manager and it looked like it
> just "cut power".

Connect to the Windows domU via rdesktop or vnc and watch what happens. :)

> 3) What is a realistic expectation for how many domU's can run on a
> host? Obviously this is dependent on how they are being used, but if
> dom0 has two dual core xeons at 2.6 GHZ and 16 GB of RAM, could you
> conceivably run 5,6,or 7 windows servers with 2 GB of RAM each? (let's
> assume NOT high transaction servers)

I haven't benchmarked it yet, or run any real performance checks, but my 
rule-of-thumb so far has been to use 1 Ghz of CPU per VM and 768 MB of 
RAM per VM as a minimum.  So, personally, I wouldn't run more than 6 VMs 
on your system without doing some benchmarking to see how thing run.

I only have two Xen boxes setup up to date:
  * 2x single-core Opteron 2 GHz CPUs w/4 GB RAM (64-bit Debian Etch dom0) 
running 4 PV VMs all running 64-bit Debian Etch domUs (these are web, 
subversion, samba, and cups servers)

  * 2x dual-core Opteron 2 GHz CPUs w/8 GB RAM (64-bit Ubuntu Hardy) 
running a mixture of PV and HVM VMs running with various FreeBSD, Linux, 
and Windows domUs, with up to 7 running at a time (this is just a test 
server at the moment)

Freddie Cash

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