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HIGH AVAILABILITY

OPTIONS IN SQL SERVER

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High Availability Definition

+ In Information Technology, high availability refers to a systemor component that is continuously operational for a desirably

long length of time

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Why we really need High Availability Options ?

+ Planned Downtime

Performing database maintenance

Performing batch operations

Performing an upgrade/update

+ Unplanned Downtime

Data center failure

Server failure

I/O subsystem failure

Human error

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High Availability Options

+ Server Clustering/Failover Clustering

+ Log Shipping

+ Database mirroring

+ Replication

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Failover Clustering

+ This provides high-availability for entire instance of sql server

+ A failover cluster is setup with a minimum of two servers,where each server is called a node that are connected to a

shared disk resource

+ Only one node is active at a time for a SQL Server instanceand serves all the requests for that instance of SQL Server

+ When the active node fails, failover takes place and one ofthe other available nodes will become the active node

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Failover Clustering(Contd.)

+ Pros

Automatic failover

Transparent client redirect

It is the only technology that allows an entire SQL Server 2008 instance

to be made highly available

+ Cons Does not protect against disk failure

More expensive since it requires special hardware for the setup as well

as redundant hardware

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Log Shipping

+ Log shipping is the simplest way to provide one or moreredundant copies of a single database.

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Log Shipping(Contd.)

+ Pros

can be configured to multiple standby servers

Can use compressed backup feature to limit network bandwidth

Can be setup to use secondary server for read only activity

No additional cost, except for the need to have another available

server for the secondary copy

+ Cons

no automatic failover

Other items need to be handled outside of Log Shipping such as logins,

SQL Agent jobs, etc... Requires additional storage for log backups and shipped copy

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Database Mirroring

+ Database mirroring provides a redundant copy of a singledatabase that is automatically updated with changes. The

purpose is to ensure continuous data availability and

minimize or avoid downtime.

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Database Mirroring(Contd.)

+ Operating Modes

High Availability

High Safety

High Performance

+ Requirement

Data and Log file locations should be same on both the principal and

mirror servers

Database name should be same on both the principal and mirror

servers Service accounts should be same on both the instances of SQL Server

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Database Mirroring(Contd.)

+ Pros

Automatic failover(only in high availability mode)

Automatic client redirection(in all the three modes)

Both server instances contain identical information(as per synchronous

operation)

Faster performance(as per asynchronous operation)

+ Cons

Doesnt support cross-database transactions and distributed

transactions

Latency can slow down operation(in case of synchronous operation) In asynchronous operation, no guarantee of identical information in

both server instances at all the times

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Replication

+ Replication feature in MS SQL Server moves the data from aremote server to our local server boxes via publications and

subscriptions mechanism with the purpose to maintain

redundant database sites.

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Replication(Contd.)

+ Types of replication

Transactional Replication

Transactional Replication with Updatable Subscriptions

Snapshot Replication

Merge replication

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Replication(Contd.)

+ Pros

Can be configured for individual database objects instead of the entire

database

Provides near real time disaster recovery

Secondary copy could be used for near real-time reporting to offload

reporting use on production database+ Cons

Reconfiguration may require manual intervention

Need to rely on distributor to push changes

Other items need to be handled outside of Replication such as logins,SQL Agent jobs, etc...