Re: [Xen-users] Re : ha cluster with virtual machines as resources

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

My comments below are aimed at paravirtualised XenLinux - HVM is a different 
kettle of fish ;-)

> >   As far as i have played with Xen , You can dynamically allocate RAM to
> > a running VM but the OS inside the VM is NOT able to detect the
> > increased RAM and therefore cannot utilize it.
> > If you reboot the VM , then on reboot it will show the increased memory.
> > ( Conditions like max mem are assumend to be satified)
>
> If I'm not completely off here, Linux PVM domUs *do* see and utilize it.
>
> Adding memory is not a big problem, reducing the amount of memory is
> one. Just imagine a process using 500 megs and you take away 200 of that
> while it is running. If you have 200 megs of unused memory, this should
> work, though.

XenLinux VM behaviour when expanding their memory is a little unintuitive.

When Linux boots, it builds a memory map - the size of this map determines how 
much memory Linux can handle.  The default is this to size this according to 
the starting size of the memory Xen gives it.  The result is that you can 
shrink the memory and regrow it back to its original size but not grow it 
beyond the original allocation.

You can pass a command line to tell Linux how much memory it should 
accommodate.  Edit the extra= section of your vm config to include "mem=4G".  
This will tell Linux that it should accommodate up to 4 gigabytes of ram, 
which is enough for most people!  If you need a different value you can 
substitute that here.

Once you've done that you can use the xm mem-max and xm mem-set commands to 
control the Xen-enforced size limit of the domain and request the domain grow 
or shrink, respectively.

Dominik has a good point, by the way: to be careful when reducing the size of 
a domain.  This can have unfortunate consequences if done overly 
aggressively.  Things can get particularly problematic if you resize a domain 
a long way below its original allocation.

Hope this helps you.  Happy resizing ;-)

Cheers,
Mark


-- 
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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