Re: [Xen-users] dom0 freezes under high IO load - HP ML150 G2

[Tim Post <tim.post@xxxxxxxxxxxxxxx>]

On Sat, 2007-03-03 at 05:57 -0300, Daniel Mealha Cabrita wrote:
> Em Sáb 03 Mar 2007 04:58, Tim Post escreveu:
> > On Fri, 2007-03-02 at 22:12 -0800, Tom Mornini wrote:
> > > You don't believe that *testing* is a good thing?
> > >
> > > I'm pretty sure the Xen documentation points out that Dom0 is not
> > > particularly special, except that it is privileged to manipulate DomUs.
> > >
> > > I'd love to hear other's opinions on this topic. Should Dom0 be
> > > entirely free of disk I/O?
> >
> > That's impossible.
> 
> I wouldn't say that.

Does dom-0 not require a base install of Linux? Does it not use
logrotate slocate brute force detection syslog sawing and other i/o
consuming needed but wasteful junk that any other server would?

> What prevents a domU from having a root in a block device exported by, let's 
> say, AoE? From dom0's perspective it would be just network traffic.

That's 100% correct. But that's in instances where one sets up a
scenario (such as AoE) where userspace in dom-0 is needed to make
dom-u's happen. The trade off to that is use a routed protocol like
iscsi and follow the ever true credence that such drivers belong in
virtual disposable machines where crashes can be recovered and modules
can easily be destroyed. 

If you follow that, then you're side swiped by the need for offload
engines and bonded gig paths defeating part of the purpose for not going
optical. Damned if you do or don't no matter what when contemplating
network storage. So, I found its best to just buy yourself a few
options.

> Now that I've mentioned, I've had heavy 'disk' dom0 problems using AoE aswell.

I've been opting for 2x 10G optical ethernet devices in most "serious"
xen farms so I can be really flexible. One becomes a routed IP network,
the other one is just used for AoE. One good 12 - 24 port switch is all
I need. Its not cheap, but no longer so expensive that its totally
prohibitive. 

Using that + 15K SAS drives has really solved 99.9% of my problems with
really demanding guests. But this isn't exactly 'off the shelf' either.

I think people just somehow lost sight of how Linux uses memory, and to
avoid needing to learn dom-0 was just named 'taboo'. 


Just treat it like any other vital system that has very little ram.
Vital systems having very little ram is not a familiar concept to many
newcomers to Linux since the days of 386 (and gasp) 286 / 8088 / 80886
users.

Best,
--Tim


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