When evaluating public and hybrid cloud services, carefully inspecting a cloud-provider’s Service Level Agreement (SLA) is an important step in understanding the level of availability you can expect.  Of course, we all want our applications and VMs to be as close to 100% available as possible, and at an initial glance, the SLA’s of many cloud-providers look deceptively similar.  When diving into the details of cloud-provider SLA’s, important differences can surface that may impact availability and cost-effectiveness of your deployed applications and VM’s in the cloud.

Comparing Cloud-Provider SLA's

In this article, we’ll compare key differences between the Service Level Agreements for Microsoft Windows Azure Infrastructure Services and the recently announced VMware vCloud Hybrid Service ( vCHS ) based on current public information provided in the SLA's for both cloud-providers as of the publication date of this article.  You can review the specific details of each SLA by following the links provided in this article.

Be sure to thoroughly review the SLA's from cloud-providers when deciding on the solution most appropriate for your applications, by asking these questions:

  • What Level of Availability is Guaranteed?
  • How is "Availability" Measured?
  • What is Excluded from SLA Guarantees?

What Level of Availability is Guaranteed?

VMware SLA … The VMware SLA for vCHS guarantees 99.9% availability for virtual machines deployed on it’s Virtual Private Cloud (VPS) shared cloud infrastructure offering.  99.95% availability guarantees are only offered for the vCHS Dedicated Cloud premium offering, which is currently priced at 2.5x – 3x the cost of VPS.  Both offerings require an upfront capacity commitment regardless of whether you are actually consuming all of the resources to which you’ve subscribed. 

Microsoft SLA … The Microsoft SLA for Windows Azure Infrastructure Services guarantees 99.95% availability when an application is hosted by two or more virtual machines that are configured as part of an availability set. In addition, VM resources can be provisioned in a cost-effective “Pay-As-You-Go” manner on-demand with no upfront commitments. 

Note that there is not an explicit SLA guarantee today for single Windows Azure VMs that are not configured in an availability set, so it's important to consider availability sets as part of your VM planning.  However, when factoring the cost of 2 VM's in a Windows Azure availability set ( needed for providing a 99.95% SLA on Windows Azure ), the associated Windows Azure compute costs appear to be less than the cost of the comparable compute resources for just a single VM using the VMware vCHS VPS shared cloud infrastructure ( which provides a lower 99.9% SLA as noted above for VPS ) based on the published pricing as of this article's publication date.

Be sure to check out the pricing comparison yourself to determine which option provides the most cost-effective availability solution for your scenarios.

  • QUESTION: Which combination of availability-level and cost-model provides you with the most flexibility and cost-effectiveness for your applications?

How is "Availability" Measured?

Some good similarities … Both vCHS and Windows Azure Infrastructure Services measure availability on a month-by-month basis.  This is important to consider, because many cloud-providers measure availability on an annual basis, which can mean that if lengthy outages across consecutive hours should occur, those cloud providers may still be within their annual SLA commitments, even if the outage is longer than what would be permitted by SLA's measured monthly.

VMware SLA … However, when reviewing the details of VMware’s vCHS SLA, it’s interesting to note that VMware considers VMs “Unavailable” only when “your running virtual machines for a class of service become inaccessible for more than 5 consecutive minutes due to physical host server failures.” Based on this definition, your applications could actually be inaccessible for less than 5 consecutive minutes several times per day, and not be considered unavailable by VMware.

Microsoft SLA … In contrast, the Microsoft Windows Azure SLA defines “Connectivity Downtime” for Virtual Machines as the “total accumulated minutes that are part of the maximum connectivity minutes that have no external connectivity.”

  • QUESTION: Which approach to measuring Virtual Machine availability would you prefer for your applications?

What is Excluded from SLA Guarantees?

VMware SLA … VMware’s SLA also declares some “interesting” exclusions to which their SLA guarantees do not apply, such as:

  • Scheduled and recurring maintenance windows
  • Denial of service attacks, virus or hacking attacks
  • Bugs in code, hardware or services

Microsoft SLA … Microsoft’s SLA declares common exclusions that are beyond the control of the Windows Azure team, such as:

  • Network or Internet connectivity issues that occur outside of the Windows Azure datacenters
  • Outages that result from a customer’s third party hardware, software or actions
  • Outages that result from trying to use operating system versions that have not been tested and found to be compatible by Microsoft
  • Use of services that have been marked as “Preview” or “Beta” and have not been rolled out for production usage.
     
  • QUESTION: Which set of SLA Exclusions seem to be more appropriate for your applications?

Are you ready to try Windows Azure?

Cloud Spectator recently released an independent study using standard Unixbench testing - Comparative IaaS Report: Analysis of 5 Large IaaS Providers - that concluded the following verbatim results regarding Windows Azure Infrastructure Services:

  • "On average, the highest-performance provider over the test period is Windows Azure, and the lowest performance provider is Amazon EC2."

  • "The difference in performance: Windows Azure scores 3 times higher than Amazon EC2 on average"

  • "Windows Azure, the highest-value provider in this scenario, provides 5x more value than on average throughout the 5-day test period than the lowest-value provider, Rackspace"

If you're evaluating IaaS Cloud Providers, be sure to leverage the resources below to try Windows Azure for FREE!

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