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High Availability with SQL 2005
‫זמינות יתרה והמשך פעילות עסקית‬
Assaf Fraenkel
Principal Consultant
Microsoft Consulting Services
Availability “Stack”
Operators
Application
Software
System
Software
Hardware
Majority of downtime is
ascribed to operators
Application Software can
have dramatic impact on
fault tolerance
System Software can help
at all levels
Hardware is very reliable at
this point
Barriers To Availability
Only some are addressable by DBMS technology
Be sure to consider people, planning, and procedures
SS 2005 gives you greatly improved
tools to overcome these barriers
Database Server Failure or Disaster
User or Application Error
Data Access Concurrency Limitations
Database Maintenance and Operations
Upgrades
Availability Solutions
How Do You Compare the Alternatives (1/2)
Time to Fail Over
Automatic or Manual Detection
Automatic or Manual Failover
Number of Failures it can survive
Data Currency / Loss
Availability Solutions
How Do You Compare the Alternatives (2/2)
Granularity of Data: Instance, Database, Table, Row
Cost of redundant system(s), additional hardware,
additional management
Complexity
Data Consistency
Transparency to clients
Availability Technologies in
SQL Server 2005
Availability Technologies in
SQL Server 2005
Basic: No failover and a potential data loss
Backup / restore
Detach / copy / attach
Better: Manual failover - potential data loss
Peer-to-Peer Replication
Log Shipping
Database Mirroring - high performance mode
Best: Automatic failover - zero data loss
Database Mirroring - high availability mode
Failover Clustering
Cold Standby Solutions
Backup / Restore and
Detach / Copy / Attach
Backup / Restore
Detach / Copy / Attach
Cold Standby Solutions
Backup / Restore and
Detach / Copy / Attach
Both Provide
Manual detection and failover
Potential for some work loss
Whole-database scope
Standard servers
Limited reporting on standby
Duplicate copy of database
Client must know where to re-connect
Slowest failover – Most downtime
Backup / Restore
What’s New in SQL Server 2005
Fine-Grained Online Repair
Online Restore – Database remains online; Only data
being restored is offline
Piecemeal Restore – Online restore of filegroups by
priority
Page-level Restore – Can restore individual pages to
repair errors found by page checksum or torn pages
Instant File Initialization – Skips file zeroing, fast
DB create / restore
Data backups do not block log backups
Restore read-only filegroups without applying logs
Backup / Restore includes FullText data
Warm Standby Solutions
Replication and Log Shipping
Replication
Log Shipping
Warm Standby Solutions
Replication
Multiple copies and Manual
failover
Primarily used where availability is
required in conjunction with scale
out of read activity
Failover possible; a custom
solution
Not limited to entire database;
Can define subset of source
database or tables
Copy of database is continuously
accessible for read activity
Latency between source and copy
can be as low as seconds
Replication
Peer-to-Peer
Transactional
Replication
Peer-to-Peer Transactional
Replication
All participants are peers
Schema is identical on all sites
Publish the updates made on ‘their’ data
Subscribe to others to pick up their changes
No hierarchy as in ‘normal’ transactional replication
A given set of data can be updated at only one site at
a time
Data ‘ownership’ is purely logical; doesn’t prevent conflicts
SQL Server prevents a change from round-tripping
Enables load-balancing and high availability
Warm/hot standby - Small possibility of data loss
Warm Standby Solutions
Log Shipping
Multiple copies and Manual failover
Basic idea: Backup, Copy & Restore
Log will always be supported
But no more investment in the scripts
Database scope
Database accessible but read-only
Users must exit for next log to be
applied
Data backups no longer block log
backups
Log Shipping
Hot Standby Failover Solutions
Failover Clustering and Database Mirroring
Both Provide
Automatic detection
Automatic, fast failover
Manual failover
Transparent client redirect
Zero work loss
Failover Cluster
Database Mirroring
Failover Clustering
Microsoft Server Cluster
Zero work loss, zero impact on throughput
Instance Failover –instance fails as a unit
Single copy of instance databases
Standby is not available for reporting,
queries, etc.
Failover Cluster
May support other instances
* Inst1
Failover Clustering
SQL Server 2005
More nodes
Match operating system limits
Supported scenarios: Multiple Active Instances,
N+1, N+I
Two-node Failover Clustering is available
in SQL Server 2005 Standard Edition
Unattended setup
N+1: N Active, 1 Inactive
N+I: N Active, I Inactive
Instances
Instances
Support for mounted
volumes (Mount
Points)
All SQL Server data
* Inst1services participate
Multiple Active Instances
Database Engine,
SQL Server Agent,
Inst2 *
Full-Text Search
Analysis Services supports multiple instances
Database Mirroring
New for SQL Server 2005
Hot Standby
Database Failover
Database Mirroring
Very fast failover
Less than five seconds in most cases
Zero data loss
Automatic or manual failover
Automatic re-sync after failover
Automatic, transparent client redirect
Database Mirroring
Fault Tolerant Virtual Database
Clients
Witness
Principal
Mirror
Witness
Witness and Quorum
Sole purpose of the Witness is
to provide automatic failover
To survive the loss of one server you must
have at least three
Witness is an instance of SQL Server 2005
Perhaps even SQL Server Express on WinXP
Consumes very little resources
Can be witness for multiple sessions
Database Mirroring
How it works
Application
Mirror is always
redoing –
it remains current
Witness
Commit
Principal
Mirror
1
5
2
SQL Server
2
Log
>2
Data
SQL Server
4
3
Log
>3
Data
Safety / Performance
There is a trade-off between performance
and safety
Database Mirroring has two safety levels
FULL – commit when logged on Mirror
Allows automatic failover
No data loss
OFF – commit when logged on Principal
System does its best to keep up
Prevents failover; to make mirror available
Must ‘force’ service
Or terminate Database Mirroring session
Database Mirroring
High Availability
Protection
Performance
Mode
Mode
Mode
Clients
X
Principal
Mirror
Witness
Transparent Client Redirect
No changes to application code
Client automatically redirected if session is
dropped
Client library is aware of Principal and Mirror
servers
Upon initial connect to Principal, library
caches Mirror name
When client attempts to reconnect
If Principal is available, connects
If not, client library automatically redirects
connection to Mirror
Database Mirroring
Impact to transaction
throughput
Database Mirroring
Depending on environment
Depending on workload
Zero to minimal
Hardware
Works with standard
computers, storage,
and networks
No shared storage components, virtually no
distance limitations
My Tips: 1GB Network, Disk for the Log
Geographically-Dispersed
Clusters and
Storage-level Replication
Geographical
Failover Cluster
Solutions from many Microsoft partners replicate
the local I/Os on a remote system
Hardware and software methods
Synchronous and asynchronous solutions
Solutions must meet Core I/O Requirements
Many solutions use MSCS to provide automatic
failover
Other solutions are primarily to duplicate the data
at a remote site, often with an independent SQL
Server
Similar to Log Shipping or Detach / Attach
Complementary Technologies
Technologies can be Combined
Replication
Maximize availability for
Scale out
Offload primary data platform
Heavy reporting
Mobile/disconnected users
Autonomous business units that share data
Failover Solutions
Maximize availability of critical systems
Designed for failover
Fast, automatic
Zero data loss
Transactionally current
Masks planned and unplanned downtime
Complementary Technologies
Failover Solution + Replication
Example
Publisher
Fault-Tolerant Publisher and Distributor
Distributor
Subscribers
Combining HA Technologies
Principal Server can be a Failover Cluster
Failover to mirror will occur before failover within the
cluster
So Principal will come back up as the Mirror
Mirror can be a Failover Cluster as well
Failover Cluster
Principal
Failover Cluster
Mirror
Barriers To Availability
As addressed in SQL Server 2005
Database Server Failure or Disaster
-----------------------------------------------------------Application or User Error
Database Snapshots
Data Access Concurrency Limitations
Database Maintenance and Operations
Upgrades
Barriers to Availability
People
I’m going to
modify this
data…right
…
here
!
This job would be great
if it weren’t for…
…the users
…the staff
…us
Database Snapshot
New in SQL Server 2005
Database Snapshots allow recovery from
user errors by allowing the database to go
back in time
Works with
Single server
Database Mirroring
Failover Cluster
Does not work with Log Shipping secondary
Database Snapshot
How it really works
CREATE DATABASE mydbSnap AS SNAPSHOT OF mydb
USE mydb
UPDATE (pages 4, 9, 10)
mydb – Database
Page
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
mydbSnap – Read-Only Database Snapshot
USE mydbSnap
SELECT (pages 4, 6, 9, 10, 14)
Database Snapshot
Technology
Extremely space efficient
Does not require a complete copy of the
data
Shares unchanged pages of the database
Requires extra storage only for
changed pages
Uses a “copy-on-write” mechanism
Database Snapshot may affect performance
on the base database
Reporting on a Mirror
Use Database Snapshots on Mirror
Database Mirroring
OLTP Clients
Witness
Mirror
Principal
Snapshot2
at 2PM
Snapshot1
at 1PM
Reporting
Clients
Barriers To Availability
As addressed in SQL Server 2005
Database Server Failure or Disaster
Application or User Error
Data Access Concurrency Limitations
Snapshot Isolation
Online Index Operations
Database Maintenance and Operations
Upgrades
Online Index Operations
Online Index Operations allow concurrent
modification of the underlying table or index
Updates, Inserts, Deletes
Online Index Maintenance
Create, Rebuild, Drop
Index-based constraints (PrimaryKey, Unique)
Data definition language (DDL) is simple
Online/Offline are both supported
Updates incur some additional cost during an
online index operation
Maintains old and new indexes
Barriers To Availability
As addressed in SQL Server 2005
Database Server Failure or Disaster
Application or User Error
Data Access Concurrency Limitations
Database Maintenance and Operations
Upgrades
Fast Recovery
Restart of a Database
SQL Server 2000
Database is available after Undo completes
Redo
Undo
Available
Time
SQL Server 2005
Database is available when Undo begins
Redo
Undo
Available
Database Maintenance & Operations
Partial Availability
Database is available if primary filegroup is
available
Online Restore
Restore while database remains available
Works with all recovery models
Backup and Restore
Data backups don’t block log backups
Full-Text Catalog is backed up and restored as
part of the database
More Operational Improvements…
Dedicated Administration Connection
Provides DBA access to server regardless of load
No server restart to kill a runaway session
More configuration is dynamic
No server restart for CPU affinity, AWE
Address Windowing Extensions (AWE)
Changes to physical size don’t require downtime
Dynamically configurable (Min / Max)
Dynamically adjusts to “hot-add” memory
AWE is available in Standard Edition (Win2003)
Instant file initialization
With appropriate security, can bypass zeroing
For create database, add file, file grow, restore
Barriers To Availability
As addressed in SQL Server 2005
Database Server Failure or Disaster
Application or User Error
Data Access Concurrency Limitations
Database Maintenance and Operations
Upgrades
Upgrade Enhancements
Software Upgrade
Re-architected – greatly reduces down time
Side-by-side installation
Resource database is pre-built
No maintenance is needed; new is substituted for old one
DBA can ignore the mssqlsystemresource.mdf and ldf
Phased
Engine and Databases (< 3 minutes)
Other components complete upgrade online after databases are
available (Replication, Workbench, etc.)
Hardware Upgrade
Hot-add memory supported without server restart
Database Mirroring minimizes downtimes for other
hardware upgrades, excluding disk
SQL Server 2005
Capabilities by Edition
Failover Clustering
Database Mirroring
Database Snapshots
Snapshot Isolation
Online Index Operations
Fast Recovery
Dedicated Admin Conn.
Table Partitioning
Log Shipping
Peer-to-Peer Replication
Enterprise
Standard
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
2-node
Safety Full
No
Yes
No
No
Yes
No
Yes
No
Workgroup,
Express
No
Witness only
No
Yes
No
No
No on Exp
No
No on Exp
No
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Assaf Fraenkel
‫הרצאות מומלצות בנושא ‪HA‬‬
‫היום ב ‪ 11:30‬עמי לוין בנושא ‪Isolaion Level‬‬
‫מחר ב ‪ 8:30‬מאיר דודאי בנושא ‪replication‬‬
Summary – What is Availability?