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Re: [Xen-devel] [PATCH] Make "xm mem-set" be lower bound on domX-min-mem

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Subject: Re: [Xen-devel] [PATCH] Make "xm mem-set" be lower bound on domX-min-mem
From: Gareth S Bestor <bestor@xxxxxxxxxx>
Date: Fri, 19 May 2006 10:10:37 -0700
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Message: 4
Date: Fri, 19 May 2006 16:29:23 +0100
From: Ewan Mellor <ewan@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] Make "xm mem-set" be lower bound on
To: "Puthiyaparambil, Aravindh" <aravindh.puthiyaparambil@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Message-ID: <20060519152923.GG27262@xxxxxxxxxxxxxxxxxxxxxx>
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On Fri, May 12, 2006 at 12:16:57PM -0400, Puthiyaparambil, Aravindh wrote:

> This patch causes "xm mem-set" to be lower bound on domX-min-mem option
> in xend-config. Another configuration option called domU-min-mem has
> been introduced, which works similarly to dom0-min-mem. This is prevents
> users from freezing the system when doing "xm mem-set" on very low
> values like 32M.
> Signed-off-by: Aravindh Puthiyaparambil
> <aravindh.puthiyaparambil@xxxxxxxxxx>

I'm not so sure about this one.  We've discussed this issue recently, and
there are a number of people (notably some of the guys at IBM) who don't want
this limit to be imposed by Xend.  If you have one VM that you know to be safe
in 16MB, say, then you either need to set domU-min-mem to 0, defeating the
point of having it, or you need to have a "force" flag to override
domU-min-mem.  The latter option here was disliked, because it was felt that
tools for VM management would just end up using the force flag all the time
because they don't know what the domU-min-mem setting is, and again, this
would defeat the point.

The min-mem value should be a per-VM setting rather than a Xend setting, so
the check belongs with the tool that's managing VMs for you.



Unless there is an known *architectural* limit in Xen on the lower bound of the memory for a guest DomU, then I agree - xend
shouldn't impose an arbitrary one simply to act as 'hard hint' to prevent stupid users from doing stupid things
(give 'em all the rope they want I say! :-) Care-and-feeding of naive users is best left to tools higher up the mgmt stack (IMO).

That said, I am very curious to understand what known or highly-recommended minimums and maximums there are for memory (and all guest resources
allocated by Xen for that matter), both for Dom0 and DomU's, so that these can be accurately reflected in the relevant min/max MemoryResourceAllocationSettingData exposed via the CIM providers.

- Gareth

Dr. Gareth S. Bestor
IBM Linux Technology Center
M/S DES2-01
15300 SW Koll Parkway, Beaverton, OR 97006
503-578-3186, T/L 775-3186, Fax 503-578-3186

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