Redundant web servers.

Bryan Sant bryan.sant at
Fri Sep 14 16:06:24 MDT 2007

On 9/14/07, Dennis <devel at> wrote:
> I've got two sites that need to have a little higher
> availability/redundancy etc.  I've been using Xen across 4 servers to
> host our sites but there isn't enough RAM to host two copies of all the
> virtual servers I've created.  (Like one copy of each server on separate
> hardware for redundancy in case one dies.)  One thing I like about Xen
> is that different servers with different dependencies don't have to have
> lots of customization.  Each one only has what it needs installed.  Xen
> has also worked great for making backups of specific servers and
> restoring or migrating them to different machines when we've had various
> hardware issues over the years. We may be outgrowing it however.

I haven't worked with Xen/OpenVZ/KVM much (yet), but VMware is
implemented with a copy-on-write memory system where two or more
virtual machines who have mostly the same memory pages, will have that
common memory mapped to the same physical RAM.  So bringing up more
VMs doesn't necessarily mean that you'll need more RAM.  I don't know
if Xen works in a similar way.  Intel VT and AMD-V technology will
push part of this feature into hardware, so it will be easy for all
virtual machine monitors to implement this feature if they haven't

I would try and see if running more VMs on your box actually takes
more physical RAM.

Anyway, it seems like all of your options are reasonable.  1 and 2
would both reduce memory usage if Xen doesn't work the way I've
described above.  However you'll be exposing the apps to each other
(which may not be a big deal).


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