99,999% availability

I sometimes hear te strangest figures when availability is discussed.

"The system must be available 95% of the time" or "The system shall never fail" or "We will only accept 99.999% uptime (5 nines)".

Usually these figures are not based on calculations and/or people have no idea about the cost of reaching these numbers.

To make things clear: All hardware will break. The question is not if something breaks, but when.

Some calculations:

There are 24*365=8760 hours in one year. 1% of this is 87,6 hours. A system with an availability of 95% can be unavailable for 438 hours per year. This means 18 full days per year!

On the other end of the horizon is the 99.999% demand. Here a system may only be unavailable for 5 minutes per year, including any repair times! The 99.999% (five nines) is a popular number these days.

Availability can be calculated by multiplying the MTBF with the MTTR.

MTBF

For hardware usually an MTBF is stated (Mean Time Between Failures). A Seagate Cheetah hard disk for instance, has an MTBF of 1.200.000 hours. This means that on average the hard disk will fail every 136 years. A system is built with many components, each with it's own MTBF. Imagine a disk cabinet with 64 disks (this is not unusual in a SAN). In such a setup, every 2 years one of these disks will fail, even with the large MTBF of the Seagate disks.

While disks are the components that fail the most (because they contain many moving parts), other components of a system also have a MTBF. For instance servers (mainly the Fans in the power supplies), routers, switches, and even cabling.

The MTBF figure is mainly a marketing instrument. How can Seagate prove that their disks will actually on average fail every 136 years? Usually this is done using simulations and tests under stress-conditions.

MTTR

Apart from MTBF, there is MTTR: Mean Time To Repair. This is the time needed to fix or replace a broken system(part). Usually the MTTR is kept low by having a service contract with the supplier of the part. Sometimes spare parts can be kept on-site to keep the MTTR low.

Software

Except for hardware, systems contain software. Usually the MTBF and MTTR for software components can not be calculated easily. No programmer will state the MTBF of the software she wrote. Who knows the MTBF of Windows? Of Linux? SAP? Your in-house developed software?

The human aspect

Usually only 20% of the causes of failures are technology failures. In 80% of the cases, human errors are the reason. For instance, a system administrator accidentally pulls a wrong cable or enters an incorrect command. Users sometimes delete inportant (system) files.

Of course it helps to have highly qualified and trained personnel, with a healthy sense of responsibility. Errors are human, however, and there is no MTBF to be calculated here.

Conclusion

As stated above, availability figures of a system are very hard to guarantee. MTBF and MTTR are either unknown, can not be calculated, or are exaggerated.

Availability can only be reported on afterwards, when a system has run for some years. With this knowledge afterwards, new systems can be designed which will probably have a higher availability.

Of course , in the last years much knowledge is gained on how to design high-available systems, for instance by using clustering, failover, redundancy, structured programming, avoiding Single Points of Failures (SPOF's) and implementing proper system management.

IT architects (or security architects for that matter) are responsible for giving availability the attention it deserves. Because the costs of being not-available can be very high, a good match between IT and business is crucial.


This entry was posted on Friday 27 October 2006

Earlier articles

Infrastructure as code

My Book

DevOps for infrastructure

Infrastructure as a Service (IaaS)

(Hyper) Converged Infrastructure

Object storage

Software Defined Networking (SDN) and Network Function Virtualization (NFV)

Software Defined Storage (SDS)

What's the point of using Docker containers?

Identity and Access Management

Using user profiles to determine infrastructure load

Public wireless networks

Supercomputer architecture

Desktop virtualization

Stakeholder management

x86 platform architecture

Midrange systems architecture

Mainframe Architecture

Software Defined Data Center - SDDC

The Virtualization Model

What are concurrent users?

Performance and availability monitoring in levels

UX/UI has no business rules

Technical debt: a time related issue

Solution shaping workshops

Architecture life cycle

Project managers and architects

Using ArchiMate for describing infrastructures

Kruchten’s 4+1 views for solution architecture

The SEI stack of solution architecture frameworks

TOGAF and infrastructure architecture

The Zachman framework

An introduction to architecture frameworks

How to handle a Distributed Denial of Service (DDoS) attack

Architecture Principles

Views and viewpoints explained

Stakeholders and their concerns

Skills of a solution architect architect

Solution architects versus enterprise architects

Definition of IT Architecture

What is Big Data?

How to make your IT "Greener"

What is Cloud computing and IaaS?

Purchasing of IT infrastructure technologies and services

IDS/IPS systems

IP Protocol (IPv4) classes and subnets

Infrastructure Architecture - Course materials

Introduction to Bring Your Own Device (BYOD)

IT Infrastructure Architecture model

Fire prevention in the datacenter

Where to build your datacenter

Availability - Fall-back, hot site, warm site

Reliabilty of infrastructure components

Human factors in availability of systems

Business Continuity Management (BCM) and Disaster Recovery Plan (DRP)

Performance - Design for use

Performance concepts - Load balancing

Performance concepts - Scaling

Performance concept - Caching

Perceived performance

Ethical hacking

Computer crime

Introduction to Cryptography

Introduction to Risk management

The history of UNIX and Linux

The history of Microsoft Windows

The history of Novell NetWare

The history of operating systems - MS-DOS

The history of Storage

The history of Networking

The first computers

History of servers

Tips for getting your ITAC certificate

Studying TOGAF

Is your data safe in the cloud?

Proof of concept

Who needs a consistent backup?

Measuring Enterprise Architecture Maturity

Human factors in security

Master Certified IT Architect

ITAC certification

Open group ITAC /Open CA Certification

Human factors in security

Google outage

SAS 70

TOGAF 9 - What's new?

DYA: Development without architecture

Spam is big business

Why IT projects fail

Power and cooling

Let system administrators participate in projects

The 7 Habits of Highly Effective People

Archimate

A meeting with John Zachman

ITAC - IT Architect certification

Personal Information is Personal Property

The Irresistible Forces Meet the Movable Objects

Hardeningscheck and hack testing for new servers

Knowledge management

Information Lifecycle Management - What is ILM

LEAP: The Redmond trip

LEAP: The last Dutch masterclasses

What do system administrators do?

Is software ever finished?

SCADA systems

LEAP - Halfway through the Dutch masterclasses

Securing data: The Castle versus the Tank

Non-functional requirements

LEAP - Microsoft Lead Enterprise Architect Program

Reasons for making backups

Log analysis - Use your logging information

Archivering data - more than backup

Patterns in IT architecture

Layers in IT security

High performance clusters and grids

Zachman architecture model

High Availability clusters

Monitoring by system administrators

What is VMS?

IT Architecture certifications

Storage Area Networks (SAN)

Documentation for system administrators

Rootkits

Presentations: PowerPoint sheets are not enough

99,999% availability

Linux certification: RHCE and LPI

IT Infrastructure model

Sjaak Laan


Recommended links

Ruth Malan
Gaudi site
Byelex
XR Magazine
Esther Barthel's site on virtualization


Feeds

 
XML: RSS Feed 
XML: Atom Feed 


Disclaimer

The postings on this site are my opinions and do not necessarily represent CGI’s strategies, views or opinions.

 

Copyright Sjaak Laan