Re: [Xen-users] Xen or KVM

["Luke S. Crawford" <lsc@xxxxxxxxx>]

On Thu, May 17, 2012 at 08:02:46PM +0200, Toens Bueker wrote:
> Mark Schneider <ms@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> I just wanted to support the other perspective to the discussion: Before
> choosing between two hypervisor-based virtualization solutions you
> should decide whether you could use os-based virtualization aka
> containers (with openvz) instead. This would spare you a lot of
> hassle, which you subscribe to by choosing a hypervisor-based solution. 

> Even the best hypervisor on the newest hardware will not give you the
> I/O (network- and storagewise), that an os-based virtualization will
> give you.
> 
> In my opinion, you should choose a solution, which will offer you "the
> best of both worlds": OS-based virtualization for the mass of your VMs
> and hypervisor-based virtualization for cases, containers can't cover.

The trouble I have with containers is that the compartmentalization?
is... much worse.   I rented FreeBSD jails before I moved to Xen,
and the Jails?  disk I/O was terrible.   I mean, total throughput
was great, far better than xen (I was using 10K disk, too)  - 
but disk IO would 'feel' slower for any particular user.

The thing is, with Xen?  the user gets his or her own pagecache.  If
they login once a week and check their mail and do nothing else?  their 
/etc/passwd /etc/shadow and their mail spool file  are all cached in ram.
The system feels very fast to the light user.   

With the Jails?   pagecache would be grabbed by the heavy user accessing
the disk a lot, so it always seemed slow for that light user.

So yeah; from my perspective?   Xen and KVM are both vastly easier
to administer than a container-based solution, as adaquate pagecache 
covers a multitude of sins.  

Of course, you are right about total/overall performance.   and in some cases,
that's the important thing.   Containers are much lighter weight.   

I just wouldn't want to manage a container that I didn't have root
(and knowledge of what to kill) on.  Xen and KVM solve the worst of the
'heavy user making it suck for everyone' problem, and make it much
easier for me to limit them without any knowledge of what it's actually 
running when it doesn't automatically take care of it.  



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