Re: [Xen-users] Why limit dom0's memory?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> Oups!!!
> total_memory           : 3322
>
> while i have 8Go of RAM...
>
> i did installed a Debian i686 on a x86_64 Server. It should not be a
> problem, isn't it?

You've not got PAE support in your Xen / XenLinux; you need that for > 4GB on 
i686.  If you can find Debian Xen and XenLinux images that support PAE 
(that's CONFIG_HIGHMEM_64G in the Linux configurator, by the way) then your 
existing install should support all the memory.

> If i save all my guests, and install again my Xen Server with Debian
> x86_64 instead of i686, then i restore the guests, will there be a
> problem?

As in xm save them?  I'm not sure if the suspend images will work on x86_64 or 
not - I've never tried / checked.  But you should be able to boot 32-bit 
guests on x86_64 /as long as you have a new enough Xen/.  I don't know what 
XEn version Debian will be running.  The guests *still* need to be PAE for 
that to work, so I suggest you investigate that route on your existing system 
first.

If you set up 32-bit PAE and then want 64-bit guests later, you can use a 
64-bit Xen with a 32-bit dom0 and that will work /as long as you have a new 
enough Xen and XenLinux/.

Hope that helps some.

Cheers,
Mark

>
>
>
> root@gaia: ~ # xm info
> host                   : gaia
> release                : 2.6.18-xen
> version                : #1 SMP Fri May 18 16:11:33 BST 2007
> machine                : i686
> nr_cpus                : 4
> nr_nodes               : 1
> sockets_per_node       : 1
> cores_per_socket       : 4
> threads_per_core       : 1
> cpu_mhz                : 2660
> hw_caps                :
> bfebfbff:20100000:00000000:00000040:000ce3bd:00000000:00000001
> total_memory           : 3322
> free_memory            : 2
> xen_major              : 3
> xen_minor              : 1
> xen_extra              : .0
> xen_caps               : xen-3.0-x86_32 hvm-3.0-x86_32
> xen_scheduler          : credit
> xen_pagesize           : 4096
> platform_params        : virt_start=0xfc000000
> xen_changeset          : Fri May 18 15:52:14 2007 +0100 15042:50fe1a769660
> cc_compiler            : gcc version 3.4.4 20050314 (prerelease)
> (Debian 3.4.3-13)
> cc_compile_by          : shand
> cc_compile_domain      : localdomain
> cc_compile_date        : Fri May 18 15:53:15 BST 2007
> xend_config_format     : 4
>
>
>
>
>
>
> On Thu, Jun 19, 2008 at 3:56 PM, Mark Williamson
>
> <mark.williamson@xxxxxxxxxxxx> wrote:
> > Hi there,
> >
> >> It's strange for me.
> >> I've got 8Go of RAM.
> >> I've defined 1024M to Dom0 on the boot (/boot/grub/menu.lst)
> >> ...
> >> kernel          /xen-3.1.0.gz dom0_mem=1024M console=vga
> >> ...
> >>
> >> I've got some Guests with some Go of RAM et it still remain a lot of
> >> RAM to use that i attibue to the news guests.
> >> But the problem that when i tried to create it, i had this error:
> >>
> >> # xm create toto.cfg
> >> Using config file "/etc/xen/toto.cfg".
> >> Error: I need 1073152 KiB, but dom0_min_mem is 200704 and shrinking to
> >> 200704 KiB would leave only 675672 KiB free.
> >>
> >> # cat /etc/xen/toto.cfg
> >> ---
> >> memory  = '1048'
> >> ---
> >>
> >> If i shutdown one of my other guest, it will ok.
> >> It's strange since i defined 1Go to Dom0, i get only 853Mo
> >> # free -mo
> >>
> >>              total       used       free     shared    buffers    
> >> cached Mem:           853        190        662          0         23   
> >>      32
> >>
> >>
> >> Would someone explain me about this?
> >> i'm with Debian Etch, Xen 3.1, 8Go RAM.
> >
> > Is that "free" output from when dom0 is freshly booted, or after you've
> > started some domains?
> >
> > Although you're setting dom0_mem=1024M, which means dom0 *initially* has
> > 1GB of RAM, it looks like your dom0-min-mem setting in
> > /etc/xen/xend-config.sxp is set to 200704, which will permit dom0 to
> > shrink in order to accommodate starting other domains.  If dom0 has
> > already shrunk then the value given by "free" will be correspondingly
> > reduced.
> >
> > The error messages you're seeing are due to dom0 not being able to shrink
> > enough to accommodate your new domain, suggesting that you're running
> > short on RAM.
> >
> > xm info can be used to find out the total amount of memory that's free
> > for Xen to allocate without dom0 needing to shrink.
> >
> > Cheers,
> > Mark
> >
> >> Thank you in advance.
> >>
> >> On Thu, Jun 19, 2008 at 4:24 AM, Mark Williamson
> >>
> >> <mark.williamson@xxxxxxxxxxxx> wrote:
> >> > That said, I think many people find that for a minimal dom0, not doing
> >> > much other work about 256MB is a reasonable amount of memory.  Maybe
> >> > you'd want to go to 512MB if you had many guests and / or memory to
> >> > spare.  dom0's requirements aren't extravagent, as long as you're not
> >> > running loads of things in it (which on a server you shouldn't, for
> >> > security reasons).
> >> >
> >> > Of course if you start running X and a modern desktop, you can expect
> >> > dom0 to have significantly higher memory requirements, just as a
> >> > normal machine would ;-)
> >> >
> >> > Cheers,
> >> > Mark
> >> >
> >> > On Monday 16 June 2008, Sandor W. Sklar wrote:
> >> >> On Jun 15, 2008, at 7:37 PM, Tim Post wrote:
> >> >> > On Sun, 2008-06-15 at 17:45 -0700, Sandor W. Sklar wrote:
> >> >> >> Well, of course, the amount of memory that a dom0 needs depends
> >> >> >> upon the service running within it.  I guess I didn't word my
> >> >> >> question explicitly enough.  Is there a formula for determine how
> >> >> >> much memory a
> >> >> >> dom0 might need, given a system with X amount of RAM, and X number
> >> >> >> of guests, assuming there are no other services running in the
> >> >> >> dom0?
> >> >> >
> >> >> > You are hoping to calculate the best possible density? I.e. give
> >> >> > dom-0 xx MB per pv guest, xx MB per HVM guest?
> >> >>
> >> >> Indeed, exactly!  Not exact numbers, but a guideline that would let
> >> >> me maximize the memory available to guests without running the risk
> >> >> of starving the dom0.
> >> >>
> >> >> > Even that is too broad to really pin down, it would really depend
> >> >> > on what you give the guests and how much they exercise the disks.
> >> >>
> >> >> OK, thanks, I guess I assumed as much, given that if such information
> >> >> existed, Google would have told me.  :-)
> >> >>
> >> >>       -s-
> >> >>
> >> >>
> >> >> _______________________________________________
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> >> >> Xen-users@xxxxxxxxxxxxxxxxxxx
> >> >> http://lists.xensource.com/xen-users
> >> >
> >> > --
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> >> >
> >> > _______________________________________________
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> >> > Xen-users@xxxxxxxxxxxxxxxxxxx
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> >>
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> >
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