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

[Tom Fritz <tfritz@xxxxxxxxx>]

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
> 12GB RAM
> 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
> each:
>
> 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is outdated (But then aren't all the xen docs).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.
http://nostarch.com/xen.htm

Good luck.

Tom.

_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users