High Availability (HA)

Minimize downtime

High Availability

High availability solutions mask the effects of a hardware and software failure and maintain the availability of system so that the perceived downtime for users is minimized.

Zero downtime is only required for highly critical systems such as GPS and live broadcasting. Excessive downtime reduction can slow down the system and increase cost due to required redundancy. For most businesses an acceptable downtime can be determined by conducting a tradeoff analysis. We consider the following items while analyzing your business requirements to determine the appropriate HA solution for your business.

  • Downtime Tolerance
  • Level of failover expectations
  • Budget
  • Scope
  • Network topology and capacity
  • Number of required secondary nodes
  • SQL Server Edition
  • Business environment
  • Database type and size
  • Expertise level of customer support team
  • Strengths and weaknesses of HA solutions

Other Advantages of High Availability solutions

  • Transfer data to other locations
  • Synchronize data among offline, online and isolated databases
  • Recover data and database
  • Handle read-only workloads such as reports, backups and BI data read
  • Decrease blocks and deadlocks by avoiding read/write contention

The five existing SQL Server HA solutions bellow can be used individually or in combination to offer different level of availability, downtime and failover functionality.

  • AlwaysOn (Available only on SQL Server 2012)
  • Replication
  • Mirroring
  • Log Shipping
  • Clustering
  • VMware HA

Tolerance for downtime

Implementation methods

  • AlwaysOn
  • Mirroring
  • Clustering


  • Quick recovery time
  • Automatic failover
  • Higher implementation cost
  • More maintenance requirement

Implementation methods

  • Replication
  • Log shipping
  • VMware


  • Medium recovery time
  • Manual failover
  • Medium implementation cost
  • Medium maintenance requirement

Implementation methods

  • Full back up
  • Differential backup
  • Transactional backup


  • Long recovery time
  • Manual failover
  • Lower implementation cost
  • Less maintenance requirement

Relationship of Availability and Downtime

The following table shows the downtime per month that will be allowed for a particular percentage of availability.

Availability % Downtime/mo.
90% 72 hours
95% 36 hours
97% 21.6 hours
98% 14.4 hours
99% * 7.20 hours
99.5% 3.60 hours
99.8% 86.23 minutes
99.9% 43.8 minutes
99.95% 21.56 minutes
99.99% 4.32 minutes
99.999% ** 25.9 seconds
99.9999% 2.59 seconds
99.99999% 0.259 seconds

(Source: Wikipedia)

* Availability limit of 99% would allow an average of 15 minutes per day which still can be handled by manual failover. Any downtime less than 15 minutes requires automatic failover.

** A widely-held but difficult-to-achieve standard of availability for a system or product is known as "five 9s" (99.999 percent) availability.