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Re: [Xen-devel] [PATCH] Add Small Memory Warning to XM for create and me

To: Anthony Liguori <aliguori@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] Add Small Memory Warning to XM for create and mem-set
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Tue, 25 Apr 2006 00:57:49 +0100
Cc: Daniel Stekloff <dsteklof@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Ewan Mellor <ewan@xxxxxxxxxxxxx>
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> I'm not sure I agree that Xend is the right place to do this.  You would
> have to add another parameter to the either the domain config or to the
> actual create() call that basically had the semantics of "if memory is
> less than X, throw an exception."  That seems like a rather awkward
> addition to the interface and it makes client code harder to write.

Surely there are already plenty of other cases in which Xend can fail a create 
and throw the result back to xm?  For instance, if the kernel image can't be 
found, hotplug scripts fail, etc.  Wouldn't it just be an extra case of that?

> I'll attempt to be telepathic here and guess that you suggested this
> because there's no xm config file so there's no easy way to be able to
> tell xm to never give an error when mem is too small?  I think this is a
> good point in time to start thinking about just implementing an xm
> config file.

Well, yes and no.

Yes, Xend is the only place to do this right now.  But we've always tried 
(modulo a few things, eg pygrub) to keep xm a pretty thin layer, with most of 
the real smarts in Xend.  This also makes it easier to create a 
network-transparent implementation of xm, without confusing Xen-related 
config files on the "control" machine and on the servers.

That said, warning about improper config choices does seem rather like a 
client-side operation so perhaps an xm.conf is worth considering.

We should also consider:
1) whether we're going to need this for other options (I suspect yes, but it'd 
be unfortunate to have a config file for just one option)
2) whether there are any config choices currently handled by Xend or the 
domain config files, which we'd rather be handled by xm.conf

> If you use SafeConfigParser and just grab from a standard location (say
> /etc/xen/xm.conf), you should be able to do it properly (just within xm)
> with very little code.

Agreed, should be a fairly simple little hack.


> Thoughts?
> Regards,
> Anthony Liguori
> > Cheers,
> > Mark
> >
> > On Friday 21 April 2006 16:17, Daniel Stekloff wrote:
> >> Hi,
> >>
> >> This patch adds a warning to xm for creating and setting a domain's
> >> memory under 32 MBs. Patch adds a "--force-mem" option to "xm create"
> >> and "xm mem-set" commands for creating or setting memory less than 32
> >> MBs. Patch also updates man page and user manual.
> >>
> >> All comments welcome.
> >>
> >> Thanks,
> >>
> >> Dan
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel

Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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