In this White Paper, we demonstrated the benefits of a consolidation of virtual machines with identical Operating Systems over the same physical hosts. The benefits are both financial and technological. The financial benefits consist of a lower license spending for Operating Systems, and increased control over spending. The main technological benefit is an increased memory sharing among Virtual Machines, which is useful to increase the VM density per physical host.

1. THE BENEFITS OF OPERATING
SYSTEMS CONSOLIDATION IN
CORPORATE DATACENTERS
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2. THE BENEFITS OF OS CONSOLIDATION IN CORPORATE DATACENTERS
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Operating Systems Consolidation
1. Introduction
In a corporate Datacenter, Operating System instances tend to span across the entire
infrastructure without a proper control.
This is an ineffective strategy, as it restrain the company from taking both the financial and
the technological advantages of Operating Systems consolidation.
Eco4Cloud offers a software architecture that improves Virtual Machines density and
resource consumption.
The Workload Consolidation module optimizes VMs placement, in order to maximize their
performances, while reducing hardware and OS licenses needs. Workload Consolidation
computes a placement that aims at consolidating VMs running the same OS onto the same
physical machines.
This lead to both financial and technological advantages: remarkable spending decrease
due to OSes licenses, and memory sharing among VMs, through VMware Transparent Page
Sharing.
2. Operating Systems Licensing
In the corporate data center, Windows and Linux-based servers work side-by-side.
The most significant characteristics for a server OS include functionality, security and
stability, as well as vendor support when things go wrong.
Most companies want the best possible reliability on servers for a good price. Linux itself is
free; the cost is for support. That is why the best Linux server OSes come from vendors
such as Red Hat and SUSE that charge for support contracts.
Overall, Linux at first look is less expensive than Windows Server because of the pricing
structure. On the other hand, Windows was thought as an all-around operating system, and
Linux does not. Data centers traditionally deploy Linux servers to offer single services;
Windows Servers are used for an all-in-one operating system that's easy to manage.
Remarkable
spending
decrease due to
OSes licenses,
and memory
sharing among
VMs

3. THE BENEFITS OF OS CONSOLIDATION IN CORPORATE DATACENTERS
© 2016 Eco4Cloud and/or its affiliates. All rights reserved. This document is Eco4Cloud Public. Page 3 of 7
With these concepts in mind, in the corporate Datacenter, the suitable alternatives are:
 Windows Server 2012 R2 Datacenter Edition
 Red Hat Enterprise Linux for Virtual Datacenters
 SUSE Linux Enterprise Server
The pricing of these OSes varies considerably.
Microsoft charges a purchase license, a one-time cost of 6155$ [1], plus a Client Access
License for each user or device accessing the server (5 CALs pack costs 189$) [2].
Conversely, Red Hat and SUSE do not charge purchase options, as Linux is free, but a
support license.
In particular, Red Hat charges an annual cost, ranging from 2499$ to 4719$ [3], while SUSE
charges an annual cost as well, but ranging from 1330$ to 2490$ [4].
All the costs are intended per single physical machine, equipped with 2 CPU sockets, and
with an unlimited number of Guest OSes supported (Fig. 1).
Figure 1. Windows Server 2012 R2 Datacenter lets you run any number of instances on a server
Therefore, there are many differences on the price structure of OSes in a corporate
Datacenter, but there is a common point: the prices depend on the number of physical
machines supporting the Guest OSes.
Eco4Cloud helps corporates increasing VMs density by 50%, and consolidating VMs running
the same OS over the same subset of physical machines, optimizing OSes cost.
Eco4Cloud helps
corporates
increasing VMs
density by 50%,
and consolidating
VMs running the
same OS over the
same subset of
physical
machines,
optimizing OSes
cost.

4. THE BENEFITS OF OS CONSOLIDATION IN CORPORATE DATACENTERS
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OS Licenses consolidation scenario
This section analyzes an example of Operating System licenses costs with and without a
proper license consolidation.
The scenario has the following assumptions:
1. The environment is a server farm consisting of 100 physical hosts
2. Microsoft licenses cost 6155$ (for the sake of simplicity, CALs are ignored)
3. Red Hat support costs 2499$ annually
4. SUSE support costs 1330$ annually
5. Without license consolidation, all physical hosts are licensed
With license consolidation, clusters are created, depending on the OS of the VMs running
on it. That allows licensing only one Operating System per physical host.
Without OS License Consolidation
Microsoft Red Hat SUSE TOTAL
year 1 $ 615.500,00 $ 249.900,00 $ 133.000,00 $ 998.400,00
year 2 $ - $ 249.900,00 $ 133.000,00 $ 382.900,00
year 3 $ - $ 249.900,00 $ 133.000,00 $ 382.900,00
TOTAL $ 615.500,00 $ 749.700,00 $ 399.000,00 $ 1.764.200,00
Table 1. OS licensing cost without license consolidation
With OS License Consolidation
Microsoft Red Hat SUSE TOTAL
year 1 $ 205.166,67 $ 83.300,00 $ 44.333,33 $ 332.800,00
year 2 $ - $ 83.300,00 $ 44.333,33 $ 127.633,33
year 3 $ - $ 83.300,00 $ 44.333,33 $ 127.633,33
TOTAL $ 205.166,67 $ 249.900,00 $ 133.000,00 $ 588.066,67
Table 2. OS licensing cost with license consolidation
As shown in Table 1 and Table 2, a proper OS license consolidation, leads to $1.176.133
cost saving in 3 years.
3. Transparent Page Sharing
VMware defines Transparent Page Sharing (TPS) as a method by which redundant copies
of pages are eliminated.
TPS is an ESX/ESXi level process, scanning every page of the guest physical memory,
searching for sharing opportunities. A full bit-by-bit comparison is performed between pages
having same TPS hash values and, if two pages match, then the guest-physical to host-
A proper OS
license
consolidation,
leads to
$1.176.133 cost
saving in 3 years.

5. THE BENEFITS OF OS CONSOLIDATION IN CORPORATE DATACENTERS
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physical mapping of the candidate page is changed to the shared host-physical page, and
the redundant host memory copy is reclaimed, thus reducing memory consumption and
enabling a higher level of memory over-commitment.
TPS works with memory pages with “regular” small pages (i.e., 4KB contiguous memory
regions), while with newer OSes and/or hosts with hardware MMU systems that make use of
large pages (i.e., 2MB contiguous memory regions), the sharing is postponed until memory
pressure happens, and ESX/ESXi breaks each large page into 2048 small pages to ease
memory swapping (and activate TPS, of course).
Put that simple, one can challenge this approach as the probability that two memory pages
composed of 4KB, that is 32768 bits, fully match is clearly infinitesimal: 1/2^32768, which is
right.
Actually, it is an established fact that when different VMs run the same OS and/or
applications, and have same data, they WILL have an amount of memory pages that fully
match, by design.
In addition, there are several situations (e.g., OS boot) where guest OS zeroes-out many
memory pages; that is, it deletes data of a each memory page by over-writing “0” on every
byte of the page. Obviously every zeroed memory page matches with the other zeroed
memory pages, and gets shared by TPS.
TPS can also lead to performance increase with reference to memory latency for large VMs
in systems composed of Non-Uniform Memory Access (NUMA) nodes. More insights on
TPS and NUMA nodes are available here and here.
Eco4Cloud strategy for TPS maximization
Eco4Cloud’s workload consolidation optimizes the energy efficiency of physical hosts by
maximizing the number of VMs running on each host, while increasing performances. In
order to do that, it computes assignment scores of VMs to hosts. A high assignment score
makes a vMotion more likely to be issued by Eco4Cloud, and the higher, the better.
Eco4Cloud’s workload consolidation is fully aware of the TPS memory reclamation
capabilities. In fact, the homogeneity of guest OSes on each physical host plays an
important role in the Eco4Cloud score assignment, increasing the odds of memory
reclamation through TPS, and leading to higher consolidation levels.

6. THE BENEFITS OF OS CONSOLIDATION IN CORPORATE DATACENTERS
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Update
In May 2015, VMware decided to disable TPS between Virtual Machines by default due to
the results of academic research, showing a technique to gain unauthorized access to data
leveraging TPS [5].
VMware believes information being disclosed in real world conditions is unrealistic, as this
technique works only in a highly controlled system configured in a non-standard way that
VMware believes would not be recreated in a production environment.
It is possible to re-enable TPS via VMware vClient.
4. Conclusions
In a corporate Datacenter, Operating Systems instances are usually deployed without a
proper control of their placement over physical hosts.
In this White Paper, we demonstrated the benefits of a consolidation of virtual machines with
identical Operating Systems over the same physical hosts. The benefits are both financial
and technological.
The financial benefits consist of a lower license spending for Operating Systems, and
increased control over spending.
The main technological benefit is an increased memory sharing among Virtual Machines,
which is useful to increase the VM density per physical host.

7. THE BENEFITS OF OS CONSOLIDATION IN CORPORATE DATACENTERS
© 2016 Eco4Cloud and/or its affiliates. All rights reserved. This document is Eco4Cloud Public. Page 7 of 7
References
[1] http://www.microsoft.com/it-it/server-cloud/products/windows-server-2012-
r2/purchasing.aspx
[2] http://www.microsoftstore.com/store/msusa/en_US/pdp/Windows-Server-2012-5-User-
Client-Access-License/productID.279149900
[3] https://www.redhat.com/apps/store/server/rhel.html
[4] https://www.suse.com/it-it/products/server/how-to-buy/shop.html
[5]
https://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&externalId=208073
5
For more information
 E4C Workload Consolidation: http://www.eco4cloud.com/workload-consolidation
 Eco4Cloud Workload Consolidation Product Overview
 Eco4Cloud Workload Consolidation FAQ
Ph. +39 0984 494276 Piazza Vermicelli
87036 Rende (CS), Italy
www.eco4cloud.com
info@eco4cloud.com