A2THOS: availability analysis and

optimisation in SLAs

Group 7:Deyang Rao

Chenhao CuiYichi Zhang

Luyang Li

Outline

• Enterprise IT Services• SLAs• Availability (FCAPS)

• Introducing A2THOS

• AND/OR dependency• Algorithm

• A2THOS prototype

• Device set-up• Performance

• Examples:• Eg.1: Oxygen• Eg.2: Hydrogen

• Reference

Enterprise IT Service

• Complicated, separated into different segments.• Commonly partly or fully outsourced to other company,

or even different companies.• Based on the basic approach: Service Level

Agreement (SLA). • Included with the Availability of certain service(s).

Availability (FCAPS)

• Availability based on Node’s dependency.• Dependency always analysed with Fault Trees (FT)

and Reliability Block diagrams (RBD).• Previous algorithm based on the Mean Time To

Failure (MTTF) and Mean Time To Restore (MTTR).• Availability = MTTF/(MTTF+MTTR)

• However the outsource service provider would not provide these data to the customer. (Who would?) So no way to calculate or estimate the availability of a higher service that depended on many basic services.

Introducing A2THOS

• A2THOS, an new approach to calculate/optimise the

availability of a certain service provided.• Based on the dependency of the infrastructure to

evaluate the system’s performance.• Improved dependency relations: AND/OR relations.

AND/OR Graph

AND Node: service would be available ONLY IF ALL sub nodes are available.

OR Node: service would be unavailable ONLY IF ALL sub nodes become unavailable.

Internal Node: a virtual node represents internal failure of the equipment.

Definition of Availability• Target Availability (What we want) [1]

• Modeling the Availability level the service has to achieve.• Variable Availability (What they provide) [2]

• Models different options with different associated price.• Given Availability (Trust your hardware) [3]

• Components’ minimal availability (Provided by hardware manufacturer)

Using components with given property [3] to build candidate systems [2] to provide

the service with a certain desired level [1] .

Prototype of A2THOS

Provide separate sub-systems for future expansion. Provide separate sub-systems for future expansion.

Benchmarks: availability analysis and optimisation algorithm

• Exponential relationship between nodes number and calculation time.

• Average 25 nodes for a mid-sized IT service.

Practical use of A2THOS

Company size:multinational company

100,000-200,000 employees

IT facilities for European branch100,000 employee workstations

many hundreds of business partners

Example 1: Oxygen

AND/OR:65 nodes, 112 edges

Example 2: Hydrogen

• Service building on-going.AND/OR： 26 nodes, 33 edgesOutsourcer offer 3 levels:

0.995/0.998/0.999Cost:

optimal combination —— cost is 2% greater 0.990 —— 30% lower

Company have to renegotiate!

Reference

I. Zambon, E., Etalle, S., & Wieringa, R. J. (2012). A2thOS: availability analysis and optimisation in SLAs. International Journal of Network Management, 22(2), 104-130.

II. Boudali, H., Crouzen, P., & Stoelinga, M. (2007). A compositional semantics for Dynamic Fault Trees in terms of Interactive Markov Chains. In Automated Technology for Verification and Analysis (pp. 441-456). Springer Berlin Heidelberg.