Re: Re: [Xen-users] Max. PV and HVM Guests

["Nick Couchman" <Nick.Couchman@seakr.com>]

Thanks for the information!  Looks like I'll be adjusting some boot-time options on my Xen servers.  I have seen a couple of issues now and then with either migration or starting a domU, but it happens once every few months at the most, and usually I blame the migration issues on a fault network connection or something like that.  I'll have to try out limiting my dom0s to 1 or 2 GB of RAM and see if those issues go away!

Thanks!
-Nick

>>> On 2009/11/09 at 08:18, Keir Fraser <keir.fraser@eu.citrix.com> wrote: 
> On 09/11/2009 15:06, "Nick Couchman" <Nick.Couchman@seakr.com> wrote:
> 
>> Really?  I thought current conventional wisdom was to allow Xen to
>> self-manage memory in both dom0 and domUs, and not to manually adjust
>> this?  I run several Xen systems with anywhere from 8 to 24 GB of RAM
>> and 20 to 30 domUs on some of these systems and have *never* specified
>> the dom0 memory at boot time - the Xen ballooning has always functioned
>> perfectly fine, and never crashed my dom0.  Furthermore, while I'm not
>> Linux developer and so not familiar with how Linux calculates buffering
>> and caching, I do know that my Linux systems dynamically manage buffers
>> and caches, and when memory is reduced or some application requires a
>> larger amount of physical memory, Linux reduces the amount of data in
>> buffers and caches.
> 
> If you are not using dom0 as a general-purpose OS then it is a very good
> idea to specify dom0's memory allowance via dom0_mem= and disable
> auto-ballooning in the xend-config.sxp. There are a few reasons for this,
> the most compelling being that Linux will have a metadata overhead for
> tracking memory usage, and this will be a fraction (say a percent or so) of
> its initial memory allocation. So, that overhead may be just 2% of 24GB,
> say, but then if dom0 gets ballooned down to 1GB it'll be more like 50%!
> Clearly you are limited in how far you can balloon down without risking the
> OOM killer in dom0.
> 
> Apart from that, the auto-ballooner has been implicated in various quirky
> bugs in the past -- failing domain creations and migrations for the most
> part -- so it's nice to turn it off if you can, as that's one less thing to
> fail. And if dom0 is single-purpose you should be able to work out how much
> memory it needs for that purpose and statically allocate it. Using
> auto-ballooner is actually perverse in this scenario, in that dom0 gets the
> least memory when it needs it the most (because it presumably has highest
> load when servicing the most VMs, but in that case auto-ballooner has stolen
> lots of memory from dom0).
> 
> My 2c!
> 
>  -- Keir




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