lowest limit for balloon?

lowest limit for balloon?

[aq]

hello,

currently there is one problem with balloon: if we specify too little
memory for a domain, that domain might crash with OOM error. even in
that case, balloon still happily do what we request. i just
accidentally balloon dom0 to 0M, and must reboot the machine ;-)

so i guess it is better to enforce a certain limit, so balloon will
not bring the amount of memory for a domain to be lower than that
level.

there are few choices to do this:
- each domain has a configurable option for this (for example:
balloon-level in domain config file)

- xen automatically determine this level for each domain (but how?)

any idea?


regards,
aq

Re: lowest limit for balloon?

[Keir Fraser]

It's probably a job for higher-level management tools. Maybe a safety 
catch should be placed on 'xm balloon', but the intra-VM 
/proc/xen/balloon interface is definitely not intended for the casual 
user.

  -- Keir


On 18 Jul 2005, at 11:57, aq wrote:

> urrently there is one problem with balloon: if we specify too little
> memory for a domain, that domain might crash with OOM error. even in
> that case, balloon still happily do what we request. i just
> accidentally balloon dom0 to 0M, and must reboot the machine ;-)
>
> so i guess it is better to enforce a certain limit, so balloon will
> not bring the amount of memory for a domain to be lower than that
> level.
>
> there are few choices to do this:
> - each domain has a configurable option for this (for example:
> balloon-level in domain config file)

Re: lowest limit for balloon?

[aq]

On 7/18/05, Keir Fraser <Keir.Fraser@cl.cam.ac.uk> wrote:
> It's probably a job for higher-level management tools. Maybe a safety
> catch should be placed on 'xm balloon', but the intra-VM
> /proc/xen/balloon interface is definitely not intended for the casual
> user.
> 

fine, but how about the balloon level to set the limit? i prefer to
calculate it based on the domain configuration, instead of specifying
it in the domain config file. but that is probably a tough job.

do you have any idea on how to determine the limit ?

regards,
aq

>   -- Keir
> 
> 
> On 18 Jul 2005, at 11:57, aq wrote:
> 
> > urrently there is one problem with balloon: if we specify too little
> > memory for a domain, that domain might crash with OOM error. even in
> > that case, balloon still happily do what we request. i just
> > accidentally balloon dom0 to 0M, and must reboot the machine ;-)
> >
> > so i guess it is better to enforce a certain limit, so balloon will
> > not bring the amount of memory for a domain to be lower than that
> > level.
> >
> > there are few choices to do this:
> > - each domain has a configurable option for this (for example:
> > balloon-level in domain config file)
> 
> 


-- 
regards,
aq

Re: lowest limit for balloon?

[Paul Larson]

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On Mon, 2005-07-18 at 13:38 +0100, Keir Fraser wrote:
> It's probably a job for higher-level management tools. Maybe a safety 
> catch should be placed on 'xm balloon', but the intra-VM 
> /proc/xen/balloon interface is definitely not intended for the casual 
> user.
I still think /proc/xen/balloon makes it far too easy to shoot yourself
in the foot, even for the non-casual user.  Simply forgetting to leave
the units off the end of an echo can bring your domain (even dom0) to a
screeching halt.

Such as doing:
echo 200000 > /proc/xen/balloon
instead of:
echo 200000K > /proc/xen/balloon.

I know that it's really difficult to determine how low is too low.
Could I recommend a small interface change though, that would force the
user to always specify units {B,K,M,...}?  That might help a little at
least.

-- 
Thanks,
Paul Larson
plars@linuxtestproject.org
http://www.linuxtestproject.org

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Re: lowest limit for balloon?

[Mark Williamson]

> I know that it's really difficult to determine how low is too low.
> Could I recommend a small interface change though, that would force the
> user to always specify units {B,K,M,...}?  That might help a little at
> least.

That sounds like it should rule out most accidental errors.  Whilst it's not 
quite so raw, /proc interfaces tend to be a bit high level anyhow...

As an alternative, I guess we could introduce a guest-internal ballooning tool 
- this could incorporate all the safety checks and user-friendliness (the 
same or similar checks should be used in the xm tool for the node admin).

Cheers,
Mark

Re: lowest limit for balloon?

[Keir Fraser]

On 18 Jul 2005, at 17:12, Mark Williamson wrote:

>> I know that it's really difficult to determine how low is too low.
>> Could I recommend a small interface change though, that would force 
>> the
>> user to always specify units {B,K,M,...}?  That might help a little at
>> least.
>
> That sounds like it should rule out most accidental errors.  Whilst 
> it's not
> quite so raw, /proc interfaces tend to be a bit high level anyhow...
>
> As an alternative, I guess we could introduce a guest-internal 
> ballooning tool
> - this could incorporate all the safety checks and user-friendliness 
> (the
> same or similar checks should be used in the xm tool for the node 
> admin).

Really, if you want to safely balloon inside a guest VM, is it so hard 
to write a tiny script to ensure a 'sane' value is passed to the /proc 
interface?

The /proc interface is only really text read/write so that accesses are 
scriptable. If you go straight at it and mess it up, I think that is 
tough luck.

  -- Keir