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Re: [Xen-users] dom0 large and in charge or petite?

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  • From: Tom Fritz <tfritz@xxxxxxxxx>
  • Date: Thu, 11 Nov 2010 17:17:34 -0600 (CST)
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On Thu, 11 Nov 2010, Scott Classen wrote:

Hello xen gurus,

I've recently acquired a new computer for the express purpose of
exploring xen. Eventually I would like to explore global shared
filesystems and the ability to migrate running domUs between different
physical computers, but for now I'm just trying to get a sense of how
many domUs I can run on a single Xeon and generally exploring the
capabilities and limitations of xen.

my test computer:
Supermico X8SAXS Motherboard.
A single Xeon L5630 2.13GHz CPU - 4 cores/8 threads
9650SE- 2LP 3ware RAID car with two 1TB drives in RAID1 configuration.
dom0 and all domUs are using the 3ware. domUs are using a disk image
file (i.e. disk = [ 'tap:aio:/var/lib/xen/images/xen1.img,xvda,w', ] )

I've installed stock CentOS 5.5 xenified kernel
(2.6.18-194.26.1.el5xen) and 4 identical domUs with 2VCPUs and 2GB RAM

dom0 = 8 VCPU 4GB RAM

xen1 = 2 VCPU 2GB RAM
xen2 = 2 VCPU 2GB RAM
xen3 = 2 VCPU 2GB RAM
xen4 = 2 VCPU 2GB RAM

My questions:
1. Since I only have 8 threads maybe is this too many domUs? Will they
be constantly fighting over CPU cycles?

The answer depends on how busy the domU's are. If they are idle most of the time then you can "over subscribe" and run many more vcpu's than you have real cpu's.

2. Should dom0 be the most minimal installation needed to administer
the domUs? Do the resources allocated to dom0 affect the performance
of the domUs?

Best practies are that you make it a minimal install. Most people will tell you that you should pin a cpu or 2 to the dom0 and allocate memory also. This can be done by adding "dom0_mem=XXXXM dom0_max_vcpus=X dom0_vpus_pin" to the kernel line in your dom0 grub config (be sure to change the X's to an appropriate value). This will insure that your dom0 doesn't compete for resources with the domU's. Like this:

kernel /boot/xen-3.4.3.gz dom0_mem=1536M dom0_max_vcpus=1 dom0_vpus_pin

3. Should I leave only 1 GB RAM for dom0 and give the rest to the domUs?

        See the above. I have found that 1-2 GB of ram works well.

Thanks, for any general advice on setting up xen.

I recommend you have a look at this book as a starting point ( I am not affiliated with the author or the publisher) It is a good general read but is outdated (But then aren't all the xen docs). It touches on most of your questions.


Good luck.


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