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Re: [Xen-users] Re: I need advice about xen project

To: Peter Booth <peter_booth@xxxxxx>
Subject: Re: [Xen-users] Re: I need advice about xen project
From: Luke S Crawford <lsc@xxxxxxxxx>
Date: 18 Sep 2009 04:31:25 -0400
Cc: Aleix Dorca <adorca@xxxxxx>, "xen-users@xxxxxxxxxxxxxxxxxxx" <xen-users@xxxxxxxxxxxxxxxxxxx>
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Peter Booth <peter_booth@xxxxxx> writes:
> For the servers running over ten domU, have you seen any issues with
> unexpected latencies or domU sar showing %steal above 1?
> Are any of these domU running web services or web applications?

Speaking as a service provider who runs a whole lot of domains one
one server,  (well, I use 32GiB/8 core servers, you can see what
sizes I provide, you can do the math.)  well, you hit lots of problems if you
don't dedicate a core to the dom0  (this means setting 
dom0_vcpus_pin=true in menu.lst, dom0-cpus 1 in /etc/xen/xend-config.sxp,
and then setting cpus=1-7 in all the DomU configs)  you see some
significant problems with I/O latency if you don't do this.  Dropped
packets, domains freezing, etc... if the Dom0 hasn't enough CPU time, 
nobody is happy.  

As for CPU contention, there's always some joker running mprime.  You 
will see significant CPU contention if you give your guests many VCPUs.
I give each guest 1 VCPU, and I don't have a problem.  Usually cpu
stays below 50% so steal has got to be pretty low.   The credit scheduler
is actually pretty good.   

Disk I/O sucks, but that's 'cause I'm putting everyone on a mirror of
7500rpm SATA disks.

Luke S. Crawford
http://prgmr.com/xen/         -   Hosting for the technically adept
http://nostarch.com/xen.htm   -   We don't assume you are stupid.  

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