1. MTBF / MTTR
Availability or recoverability?
Presented by
Michael Richardson, Energized Work
21 March 2012
ENERGIZED WORK
25 MACKLIN STREET
LONDON WC2B 5NN
+44 (0)20 7691 8933
WWW.ENERGIZEDWORK.COM

2. Michael Richardson
Twitter: @mr_spb
Email: michael@energizedwork.com
#ewtektalk
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3. So what is
high availability?
• Five nines?
• No single point of failures?
• Multiple data centres?
• Fault tolerance?
• Load balancing?
• Uptime?
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4. Nines
of availability
9 9
9
9
9
9
9 9
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5. Nines
of availability
Availability
Downtime per Year
One nine (90%)
36.5 days
Two nines (99%)
3.65 days
Three nines (99.9%)
8.76 hours
Four nines (99.99%)
52.56 minutes
Five nines (99.999%)
5.26 minutes
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6. Problem with
the nines
• What do they mean?
• Guaranteed or just an SLA?
• Multiplicity (99.9% * 99.9% * 99.9% = 99.7%)
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7. SLA availability numbers
just aim to provide a level of
confidence in a website’s service
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8. No single point of failure
(SPOF)
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9. Two of everything?
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10. Start with this
Users
Index.html
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11. End with this
Users
Firewall 1 Firewall 2
Switch 1 Switch 2
WEB1 WEB2 APP1 APP2 DB1 DB2
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12. Problems with
eliminating SPOF
• It’s expensive
• Where do you draw the line?
• Are failures independent?
• Can you guarantee no SPOF?
• Increased complexity
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13. Problem:
Data centres fail
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14. Solution:
Get a second data centre
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15. Hot – Hot
multisite
• Full range of services available in multiple locations
• Easy to automate failover of sites
• Data consistency is hard
• Capacity planning concerns
+
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16. Hot – Warm
multisite
• Simpler than hot – hot
• Read / Write ratio dependent
• Synchronously or asynchronously replicate data?
+
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17. Hot – Cold
multisite
• Easy to setup
• Will it work?
• Can it be trusted?
• Cold site rapidly becomes stale
• Is it actually valuable?
+
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18. DR multisite
• Fingers crossed you never need it
• How can / should you test it?
• Cloud?
+
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19. Problems
with multiple sites
• It’s expensive
• Managing more systems
• Managing data consistency
• Managing capacity
• Is it still fail proof?
• Unless you test it, it’s just a plan
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20. We now have
a complex system
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21. Complex systems
• More redundancy and automation leads to more complexity
• More complexity often adds more points of failure
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22. How complex systems fail
- Dr. Richard Cook
• Catastrophe is always just around the corner
• Human operators have dual roles
• Change introduces new forms of failure
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23. Failure and recovery
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24. Questions
for the business
• What is the cost of downtime?
• What are the Recovery Time Objectives (RTO)
• What are the Recovery Point Objectives (RPO)?
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25. Aggressive RTO and RPO
are expensive and have a
performance impact
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26. RTO / RPO
example
Problem:
• Simple DB
• Business can tolerate up to 15 minutes downtime
• 10-minute window of data loss
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27. RTO / RPO
example
Possible solution:
• Continuously replicate data to second host
• Continue with nightly backups and also copy DB transaction logs
from the primary host to another system
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28. So what is more important –
increasing availability
or reducing recovery time?
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29. MTBF or MTTR?
What about MTTD?
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30. The answer is:
It depends
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31. Failure
is inevitable
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32. Ask anyone
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ENERGIZED WORK
25 MACKLIN STREET
LONDON WC2B 5NN
+44 (0)20 7691 8933
© 2012 Energized Work - www.energizedwork.com WWW.ENERGIZEDWORK.COM
33