The Health Model, Service Class, and Scale Unit patterns directly affect Infrastructure and are detailed in the relevant sections later. Conversely, private cloud infrastructure design directly affects Physical Fault Domains, Upgrade Domains, and the Cost
Model. These relationships are shown in Figure 2 below.
Figure 2: Infrastructure Relationship with Patterns
The private cloud principles “perception of continuous availability” and “resiliency over redundancy mindset” are designed to make a private cloud architect think differently.
Traditional solutions rely heavily on redundancy to achieve high availability and avoid failure. But redundancy at the facility (power) and infrastructure (network, server, and storage) layers is very costly. Modern cloud applications are designed with a different,
holistic approach to achieving availability. This means shifting focus from building redundancy into the facility and physical infrastructure to engineering the entire solution to handle failures -- eliminating them, or at least minimizing their impact.
This approach to availability relies on resilience as well as redundancy. Resilience means rapid, and ideally automatic, recovery from a failure. Redundancy is typically achieved at the application level. (A non-cloud example is Active Directory®, where redundancy
is achieved by providing more domain controllers than is needed to handle the load.)
Customer interest in cost reduction will help drive adoption of this approach over the medium term. Removing power redundancy from racks or co-location rooms has a big impact on operational expenses, but this typically occurs only when the hosted application
doesn’t have to be highly available, or when high availability is achieved through redundancy at the application layer – for example, Active Directory replication, or application layer mirroring such as SQL Server™ mirroring. Combining reductions in physical
redundancy with virtualization results in lower capital and operational expenditure compared to a highly redundant infrastructure.
Applications that depend on a highly available infrastructure will not achieve their Service-Level Agreement (SLA) when placed on the type of infrastructure defined earlier. Customers are therefore likely to develop two environments when designing their private
cloud: a standard environment with reduced facility and infrastructure redundancy, and a high-availability environment with traditional levels of redundancy.
No power redundancy to the rack (for example one in-rack UPS)
These two environments allow a Architect to differentiate service classifications from a high-availability perspective. The standard environment is appropriate for stateless workloads; stateful workloads will require the high availability environments. Stateful
and stateless machines are managed differently. Statefulness will likely appear as a characteristic of the service classifications.
Stateless workloads (web servers, for example) are typically redundant at the server level via a load-balanced farm. These servers could easily be hosted in the Standard Environment. If all stateless workloads had an automated build, the Standard Environment
could do away with any form of VM migration – and simply deploy another VM after destroying the existing one, thereby saving the cost of shared storage.
Stateful workloads, on the other hand, require a specific management approach and impose higher costs on the consumer. Unless designed for high availability at the application level, they will require some form of redundancy in the infrastructure. Further,
the High-Availability Environment requires Live Migration to enable maintenance of the underlying fabric and load balancing of the VMs.
The number one concern of customers considering moving services to the cloud is security. Recent concerns expressed in the industry forums are all well founded and present reasons to think through the end to end scenarios and attack surfaces presented when
deploying multiple services from various departments in an organization on a private cloud.
In a cloud-based platform, regardless of whether it is private or public cloud, customers will be working on an essentially virtualized environment. The platform or software will run on top of a shared physical infrastructure managed internally or by the service
provider. The security architecture used by the applications will need to move up from the infrastructure to the platform and application layers. In private cloud security this will provide security in addition to the perimeter network.
Public cloud involves handing over control to a third party, sharing services with unrelated business entities or even competitors and requires a high degree of trust in the providers security model and practices. In many ways the security concerns of private
cloud and similar those of self-hosted or outsourced datacenter however the move to a virtualized self-service service oriented paradigm inherent in private cloud computing introduces some additional security concerns.
First is the isolation of tenants from each other and the hosting infrastructure at both the compute and network layers. Virtualization is a part of any private cloud strategy and the security of this model is totally dependent on the ability to isolate one
tenant from another and prevent the careless or malicious tenant from impacting the stability of the core infrastructure upon which all tenants rely.
Another concern is Authentication, Authorization and Auditing of access to the cloud services. Self-service implies that tenant administrators can initiate management processes and workflows where previously this was accomplished through IT. For any misconfiguration
or excessive permissions granted to these users can impact the stability or security of the cloud solution.
Many private cloud security concerns are also shared by traditional datacenter environment which is not surprising since the private cloud is just an evolution of the traditional datacenter model. These include:
The biggest risk in running in a multi-tenant virtualized environment is that a tenant running services on the same physical infrastructure as you can break out of its isolating partition and impact the confidentiality, integrity or availability of your
workload and data. Therefore the security in virtualization platform is key in the isolation and non-interference between the individual virtual machines running on the infrastructure.
Many management tasks involve multiple steps that must be completed in the proper sequence by multiple administrators across multiple systems. Any shortcuts, omissions or errors can leave assets vulnerable to unauthorized access or affect the reliability
of components within a solution. By orchestrating discreet management and monitoring tasks into workflows that require proper authorization and approval greatly diminish the chance of mistakes that affect the security of the solution.
Most organizations have a common capability for providing an overarching framework for authentication and access control and then a private cloud introduces all parts of hosting and hosted services that include the hosting infrastructure and the virtual
machines workloads that run in that infrastructure. This framework must be designed and possibly extended to provide a single point of managing identities and credentials, authentication services and common security model for access to resources across the
Moving to a cloud-based platform requires a change in mind-set of developers and IT security professionals. Some of the risks of the public cloud are mitigated by using a private cloud architecture, however, the perimeter security protecting a private cloud
should be seen as an addition to public cloud security practices, not an alternative. You cannot apply the traditional defense-in-depth security models directly to cloud computing, however you should still apply the principal of multiple layers of security.
By taking a fresh look at security when you move to a cloud-based model, you should aim for a more secure system rather than accepting security that continues with the current levels.
Enterprise IT systems are now typically well regulated and controlled. The security risks are well documented and therefore proper processes are put in place to develop new applications and systems, or to provision them from 3rd party vendors. It is very
unlikely that a department manager would be able to purchase and install software without approval from the IT department.
With public cloud systems and Web browser clients however, it is possible that individual department managers could bypass the IT department and provision public cloud-based software. Indeed, they might use free cloud storage systems as a convenient means to
synchronize documents without even considering that they are using public cloud services. Public cloud systems might be appealing to a manager as they could very quickly provision a new system and remove what they might see as unnecessary bureaucracy. They
may even be unaware of the security and compliance policies that are in place to protect the organization. In a cloud-based landscape, we must protect corporate systems and data from these unauthorized, untested systems.
Facilities represent the physical components – buildings, racks, power, cooling, and physical interconnects – that house or support a private cloud. It is beyond the scope of this document to provide detailed guidance on facilities, but the private cloud principles
affect facility design.
The definition of a Scale Unit impacts power, cooling, space, racking, and cabling requirements. The team that defines a Scale Unit should include personnel that design and manage these aspects of the facility in addition to the procurement, Capacity Planning,
and Service Delivery teams. The following table lists some ramifications of Scale Unit size choices from a facilities perspective.
Small Scale Unit
Large Scale Unit
Knowing how much power, cooling, and space each Scale Unit will consume enables the facilities team to perform effective Capacity Planning and the engineering team to effectively plan resources.
The term Fabric defines a collection of interconnected compute, network, and storage resources.
The concept of homogenous physical infrastructure, introduced in the Private Cloud Principles, Concepts, and Patterns guide, stipulates that all servers in a Resource Pool should
be identical. Homogenizing the compute, storage, and network components in servers allows for predictable scale and performance. In other words, every server in a Resource Pool should have the same processor characteristics such as family (Intel/AMD), number
of cores/CPUs, and generation (Xeon 2.6 Gigahertz (GHz)). The homogenized compute concept also stipulates that each server have the same amount of Random Access Memory (RAM) and the same number of connections to Resource Pool storage and networks. With these
specifications met, any virtualized service could relocate from one failing or failed physical server to another physical server and continue to function identically.
The physical server hosts the hypervisor and provides access to the network and shared storage. In the Standard Environment, the facilities do not provide power redundancy, so the servers do not require dual power supplies.
Every server will be a member of a single compute Resource Pool and a single Physical Fault Domain. Assuming all servers are homogeneous (as recommended), they will all be members of a single Upgrade Domain.
Capacity Planning must be done for each server specification, as its size (CPU and RAM specification) will determine how many virtual machines it is able to host. This is covered in greater detail in the Private
Cloud Planning Guide for Service Delivery.
Server specification selection impacts the Scale Unit, Cost Model, and service class. Scale Units have a finite amount of power and cooling, so server efficiency has an impact on a private cloud. It may be that all power in a Scale Unit is consumed before all
physical space. The cost of servers impacts the Cost Model irrespective of whether this cost is passed onto the consumer. Selecting only small one-unit servers will limit the architect’s ability to define a range of service classifications. The server needs
to accommodate the largest service classification after the parent partition and hypervisor consume their resources.
Microsoft research shows servers with processors one or two models behind the latest versions offer a better price, performance, and power consumption ratio than the newer processors.
The Private Cloud Reference Architecture dictates that the “concept of homogenization of physical infrastructure” be adopted for each Resource Pool. Server specifications (CPU, RAM) may vary between Resource Pools, but this complicates Fabric Management (defined
Private Cloud Planning Guide for Systems Management), which spans Resource Pools and Capacity Planning, and may necessitate different service classes for each pool.
Delivering IaaS requires that the service is pre-defined and delivered consistently. To achieve consistent performance, the VMs must have equal resources available to them from each server, in other words, the same CPU cycles and RAM. If servers within a Resource
Pool do not provide homogeneous performance and RAM, consistent performance cannot be guaranteed.
Absolute homogenization may be hard to maintain over the long term as server models may be discontinued by the vendor; therefore relationships between Resource Pools, Scale Units, and server model longevity must be considered carefully.
The following table lays out some of the benefits and trade-offs of homogeneous and heterogeneous Resource Pools.
Homogeneous Physical Infrastructure
Heterogeneous Physical Infrastructure
In addition, servers should support the following requirements to achieve an automated infrastructure and resiliency:
To dynamically initiate remediation events in case of failure or impending failure of server components, each server is required to display warnings, errors, and state information for the following:
To achieve the perception of infinite capacity, proactive Capacity Management must be performed, and storage capacity added ahead of demand. The amount of storage added as a single unit (a Storage Scale Unit) will depend on the rate of storage consumption,
hardware vendor lead time, and the level of risk the business wishes to assume (that is, weighing remaining unallocated capacity against the possibility of exhausting all capacity). This is detailed in Private
Cloud Planning Guide for Service Delivery.
Storage will be placed in Storage Resource Pools, from which it is automatically allocated to consumers. Though Resource Pools are not a new concept for Storage Area Networks (SANs), allowing the infrastructure to allocate storage on-demand based on policy
may be a new approach for many organizations. Further, the SAN must present an application programming interface (API) to Fabric Management to allow automation of allocation and provisioning.
The storage provided within a private cloud must be consistent in performance and availability. This means the Input/output (I/O) Operations per Second (IOPS) cannot vary significantly. If there is a need to make different levels of storage performance available
to users of a private cloud, it can be accomplished through multiple service classifications. A private cloud is intended, however, to provide a limited set of standardized services; therefore, variances should be carefully considered.
The cost of providing the storage within a private cloud should be clearly defined. This permits metering, and possibly allocation of costs to consumers. If different classes of storage are provided for different levels of performance, their costs should be
differentiated. For example, if SAN is being used in an environment, it is possible to have storage tiers where faster Solid State Drives (SSD) are used for more critical workloads. Less-critical workloads can be placed on a Tier 2 Secure Attention Sequence
(SAS), and even less-critical workloads on Tier 3 SATA drives.
The Private Cloud Reference Architecture assumes the storage arrays and the storage network are redundant, with no single point of failure beyond the array itself. In this regard, the storage array can be considered a Fault Domain.
The design should adopt some form of de-duplication technology to reduce storage consumption.
As the storage array is a single point of failure, it should display health information to the systems monitoring service to make sure that any outages and their impact are quickly identified. Providing snapshots and mirroring between arrays for continuity
is beyond the scope of this guidance.
If a Architect follows the recommendation to allow any VM to execute on any server in a Resource Pool, Virtual Hard Disks (VHDs) should reside on a SAN. While it is possible to host VHDs locally, the guidance assumes that they are hosted on a SAN.
A key decision in private cloud design is whether to use iSCSI or Fiber Channel for storage. If iSCSI is utilized to house virtual workload storage, it is suggested that each virtualization host include iSCSI HBAs instead of standard NICs for performance reasons.
The purpose of a storage switch is to provide resilient and flexible connectivity between shared storage and physical servers. The storage switch must meet peak storage I/O requirements for the virtual services. In addition, the interconnect speeds between
switches should be evaluated to determine the maximum throughput for switch-to-switch communications. This may limit the maximum number of hosts that can be placed on each switch.
While switch throughput is important, attention should also be paid to the number of available switch ports needed to support the physical virtualization hosts. Refer to the switch hardware vendor to make sure it meets these requirements.
Physical storage switch requirements include:
Stateless workloads can be hosted on Direct-Attached Storage (DAS) instead of SAN, driving down the cost of service. The downside is that Fabric Management has to handle transitioning active user connections between VMs homed on different hosts, as VM migration
is impossible. This may mean tighter integration with the network than is specified in this document (in order to know when all connections to a VM have been abandoned or terminated before stopping the VM, for example).
SAN storage, while more expensive, provides advantages:
The logical storage configuration (or storage classification) should be designed to meet requirements in the following areas:
Additionally, a private cloud must meet the following requirements to make sure that it is highly available and well-managed:
The Private CLoud Reference Architecture assumes that the network presented to servers is not redundant for the Standard Environment and is redundant for the High-Availability Environment.
The network is tightly coupled with physical servers. Each Compute Resource Pool includes the network switches necessary for the servers to operate; each Scale Unit includes a pre-defined and fixed number of servers and switches.
The switches must be monitored to make sure no workloads saturate the network. A private cloud is designed as a general-purpose infrastructure. Workloads that challenge the network with high utilization may not be good candidates for a private cloud unless
separate Resource Pools are created specifically to handle these workloads.
Switches are members of network upgrade domains, but the definition and membership of upgrade domains will likely vary depending on the nature of the upgrade. If switches are not redundant (for example, in the Standard Environment), the whole Resource Pool
will need to be taken offline for switch maintenance, which requires switch reboots.
Network hardware (switches and load balancers) must display an API to Fabric Management that enables automated management of networks such as creation of VLANs, Virtual IP addresses (VIP), and addition or removal of hosts from the VIP.
Some key decisions that should be made to increase the bandwidth of the physical networks are related to the use of Live Migration requirements of port security, and the need for link aggregation. Here is a table showing the benefits and trade-offs of using
Use Live Migration
Do Not Use Live Migration
To support the dynamic characteristics of a private cloud, a network switch should support a remote programmatic interface – for firmware upgrades, and prioritization of traffic for quality of service. These switches should be dedicated for a private cloud to
maintain predictable performance and to minimize risks associated with human interaction. As defined earlier, the servers need to be connected to at least two networks, management and consumer, with live migration (if required). The connections should always
be the same; for example, network adapter 1 to management, network adapter 2 to consumer, and network adapter 3 to Live Migration.
If iSCSI is chosen for the storage interconnects, iSCSI traffic should reside in an isolated VLAN in order to maintain security and performance levels. This iSCSI traffic should not share a network adaptor with other traffic, for example the management or consumer
The interconnect speeds between switches should be evaluated to determine the maximum bandwidth for communications. This could affect the maximum number of hosts which can be placed on each switch.
When designing network connectivity for a well-managed infrastructure, the virtualization hosts should have the following specific networking requirements:
To dynamically initiate remediation events in response to the failure or impending failure of network switch components, each switch is required to display warnings, errors, and state information for the following:
To dynamically initiate remediation events in response to either the failure or impending failure of storage switches and storage subsystem components, each component is required to display warnings, errors, and state information for the following:
The hypervisor exposes the VM services to consumers. It needs to be configured identically on all hosts in a Resource Pool, and ideally all hosts in the private cloud. Fabric Management will orchestrate the addition of virtual switches, machines, and disks.
An architect needs to decide whether the private cloud should use CPU Resource Reservations to make sure of predictable performance of VMs. This table lists the benefits and trade-offs:
Use CPU Resource Reservations
Do Not Use CPU Resource Reservations
The decision is driven by whether efficiency or consistency is more important for the private cloud.
The architect could elect to provide different classes of services – one which uses resource reservations to deliver predictability, and another which shares the resources. Separate Resource Pools could be deployed accordingly, along with differential pricing
to incent the consumers to exhibit desired behavior.
Resource reservations will not prevent a host from saturating the network and crippling the performance of other hosts. As stated in the Network section earlier, this needs monitoring.
The parent partition provides the hypervisor with access to physical resources such as network and storage. It also hosts the hypervisor management interfaces. The parent partition needs to be configured identically on all servers in a Resource Pool.
If an architect elects to create a service classification which depends on consuming LUNs directly (not via the parent partition), the parent partition must be configured to present the pass-through for this storage. Further, this storage must be available
to all parent partitions in that Resource Pool to enable VM portability between hosts.
The parent partition displays health information for the server, the parent partition operating system, and the hypervisor. The health monitoring system, in turn, consumes this information to enable Capacity Management and Fabric Management.
Task execution is the low level management operations that can be performed on a platform and generally are surfaced through the command line or Application Programming Interface (API). The capability to execute tasks must not only exist but the usage semantics
should be consistent across members of a fault domain to enable automation using a common format. When differences in semantics exist this forces the automation layer to compensate for these differences through custom code in the orchestration or even require
using different execution hosts or engines within a fault domain.
The automation layer is made up of the foundational automation technology plus a series of single purpose commands and scripts that perform operations such as starting or stopping a virtual machine, restarting a server, or applying a software update. These
atomic units of automation are combined and executed by higher-level management systems. The modularity of this layered approach dramatically simplifies development, debugging, and maintenance.
In much the same way that an enterprise resource planning (ERP) system manages a business process such as order fulfillment and handles exceptions such as inventory shortages, the orchestration layer provides an engine for IT-process automation and workflow.
The orchestration layer is the critical interface between the IT organization and its infrastructure and transforms intent into workflow and automation.
Ideally, the orchestration layer provides a graphical user interface in which complex workflows that consist of events and activities across multiple management-system components can be combined, to form an end-to-end IT business process such as automated patch
management or automatic power management. The orchestration layer must provide the ability to design, test, implement, and monitor these IT workflows.
Service management provides the means for automating and adapting IT service management best practices, such as those found in the IT Infrastructure Library (ITIL), to provide built-in processes for incident resolution, problem resolution, and change control.
Self Service capability is a characteristic of private cloud computing and must be present in any implementation. The intent is to permit users to approach a self-service capability and be presented with options available for provisioning in an organization.
The capability may be basic where only provisioning of virtual machine with a pre-defined configuration or may be more advanced allowing configuration options to the base configuration and leading up to a platform capability or service.
Self service capability is a critical business driver that enables members of an organization to become more agile in responding to business needs with IT capabilities to meet those needs in a manner that aligns and conforms with internal business IT requirements
This means the interface between IT and the business are abstracted to simple, well defined and approved set of service options that are presented as a menu in a portal or available from the command line. The business selects these services from the catalog,
begins the provisioning process and notified upon completions, the business is then only charged for what they actually use.
This is analogous to capability available on Public Cloud platforms.
The entities that consume self service capabilities in an organization are individual business units, project teams, or any other department in the organization that have a need to provision IT resources. These entities are referred to as Tenants. In a private
cloud tenants are granted the ability to provision compute and storage resources as they need them to run their workload. Connectivity to these resources is managed behind the scenes by the fabric management layers of the private cloud.
Tenant administrators are granted access to a self-service portal where they can initiate workflows to provision virtualized services in the appropriate configuration and capacity. For example compute resources may be available in small, medium or large instance
capacities and also storage of the appropriate size and performance characteristics. Resources are provisioned without any intervention from infrastructure personnel in IT and the overall progress is tracked and reported by the fabric management layer and
reported through the portal.
A chargeback model is defines how tenants will be charged for using the cloud resources. This is typically the numbers and size of resources provisioned times the amount of time they are provisioned for. This information is available to tenant administrators
through the self-service portal and well as the ability to provide cost reporting.
Tenants are granted the ability to manage, monitor and report on the resources that they have provisioned.
If you edit this page and would like acknowledgement of your participation in the v1 version of this document set, please include your name below:
[Enter your name here and include any contact information you would like to share]
Return to Reference Architecture for Private Cloud
very good article i like it so much !
In the Storage Subsystem Health Model outline I would have expected to see something on Logical Disk Groups with respects to state (Online/Degraded) and Used/Free space.