Coeo SQL Server 2008 High Availability

7/29/2019 Coeo SQL Server 2008 High Availability

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Justin Langford

Principal Consultant


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The business continuity problem

Cold/ Warm/ Hot Standby Solutions

Failover Clustering

Database Mirroring

Combining Availability Technologies


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Fundamentals No failover, potential data lossBackup and RestoreThird party solutions: storage snapshots, VDI

Detach/ Attach

Distributed Data Manual failover, possible data lossLog ShippingPeer-to-peer replication

Database Mirroring (High Performance mode)

Data Mirroring (third party hardware/ software mirroring

High Availability Automatic failover, no data lossDatabase Mirroring (High Availability mode)

Failover clustering

Multi-site (Geo) clusters


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Application

Software

System

Software

Hardware

Majority of downtimeattributed to System Admins

Application Software can

improve fault tolerance

System Software can help atall levels

Commodity hardware nowvery reliable

Operator


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we need zerodowntime,zero data

loss

Evaluating business continuity solutions

Automatic or Manual DetectionAutomatic or Manual FailoverFailover timeNumber of failures survivedData Currency / LossData ConsistencyGranularity: Instance, Database, Table, Page, Row

Cost - hardware, network, additional managementComplexity

Recovery Point Interval/ Recovery Time Interval


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Transparency to clients


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Failover Clustering

Database Mirroring



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Manual detection and failoverPotential for data lossClients must be redirected

Slowest failover most downtime

Backup / Restore Log backups allow point in time restore

Detach / Copy / Attach Copies entire files No rolling forward logs

Backup / Restore

Detach / Copy / Attach

Backup/ Restore & Detach/ Attach


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Manual detection and failover

Replication since SQL Server 6.0

Primarily used where availability is required in

conjunction with scale out of read activity

Failover requires custom solution Could define subset of source database

Latency between source and copy can be seconds

Log Shipping

Basic idea: Backup, Copy, Restore Log will always be

supported

Database scope

Database accessible but read-only

Users must exit for next log to be applied

Log Shipping

Replication

Replication and Log Shipping


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SQL Server 2005 introduced bi-directional

transactional replication

Enables scale-out and improved availabilityWarm / hot standby Possibility of data loss on failure

SQL Server 2008 improvements

Conflict detection

Each row has a hidden column listing originating peer node ID for change

Distribution Agent on each node detects conflicts by comparing hidden column

New Topology Wizard makes setup/ changes easier

Peer-to-PeerTransactionalReplication

Peer-to-Peer Transactional Replication


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Both solutions provide Automatic detection Automatic, fast failover

Manual failover Transparent client redirection Zero work loss Site redundancy

Database Mirroring

Failover Cluster

Failover clustering and Database Mirroring


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Failover Clustering

Database Mirroring



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Hot Standby Automatic detection and failoverMultiple nodes provide availability

Failover transparent to client

Windows Server 2008 EE supports 16 node clusters

Windows 2008 clusters must pass validation tests (no HCL)

Supports many scenarios: Multiple Active Instances, N+1, N+I

N+1: N Active, 1 Inactive

Instances

Inst2 *

* Inst1

Multiple Active Instances

Inst2 *

* Inst1

Inst3 *

N+I: N Active, I Inactive

Instances

Failover Cluster

* Inst1

SRV1 SRV2


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Node Majority

Each node has a vote

Cluster functions with majority of votes

Node and Disk Majority

Each node plus the disk witness can vote


Node and File Share Majority

Each node plus the file share witness can vote


No Majority: Disk Only

Cluster functions if one node is available with quorum disk presented

Nodes in communication with quorum disk are cluster members

Failover Cluster

* Inst1

Majority

meansmore than

half

F il Cl


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Choosing a Quorum Model

Failover Cluster

* Inst1

Description of Cluster Quorum Recommendation

Odd number of nodes Node Majority

Even number of nodes (single site) Node and Disk Majority

Even nodes, multi-site cluster Node and File Share Majority

F il Cl t


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Zero work loss, minimal impact on throughput

Instance Failover entire instance works as a unit

Single copy of databases

Available since SQL Server 7.0

No standby reporting, testing etc.

Single cluster can support multiple instances

Can provide site fault-tolerance

Failover Cluster

* Inst1

F il Cl t


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No remote execution on cluster nodes

Run SQL Server setup to Install/ uninstall / upgrade nodes

Option 1: Integrated installation with Add Node

Configure single-node cluster instance

Run SQL Server Setup with Add Node functionality to add each node

Option2: Advanced/Enterprise installationRun Setup with Prepare Failover Cluster on each node

Run Setup on node owning shared disks to Complete Failover Cluster

Failover Cluster

New setup for SQL Server 2008


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Site redundancySQL Server requires single-subnet

Stretch VLAN between sites

Requires synchronous storage replication

Performance dependent on network RTT

Configure Heartbeat timeout

Increase to extend clusters over further distances

Decrease to increase failure sensitivity for fasterfailover

Two Quorum configurations suitable for multi-sitefailover clusters


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STORAGE REPLICATION

Site B

SAN

\\SERV1\CLUS1

File Share

Witness

Site C

Site A

SAN

Node and File Share Majority


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Site B

SAN

Site A

SAN

Node Majority

STORAGE REPLICATION


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Failover Clustering

Database Mirroring


Database Mirroring


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Hot StandbyProvides a fault-tolerant database

Database Failover

Very fast failover


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MirrorPrincipal

Witness

Log

Application

SQL Server SQL Server

2

2

4

51

Data DataLog

3>2 >3

Mirror alwaysredoing log toremain current

Commit


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High Availability - synchronous mirroring with a witness

Automatic detection/failover

No data loss

High Protection - synchronous mirroring without a witness

Manual failover

No data loss, downtime possible

High Performance - asynchronous mirroring

Manual failover

Data loss possible

Trade off: Performance vs. Safety


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Available in SQL Server 2008 Enterprise EditionAutomatic Page Repair

Auto-resolve some errors that prevent reading a page

Retrieve fresh copy from partner

Unreadable page is replaced by the copy

Repairs are asynchronous

Query on principal accessing corrupt page will fail

If mirror accesses corrupt page - mirroring session issuspended


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Not all errors can be repaired

823, 824 errors - Windows returns a CRC error when reading page

829 errors restore pending

Pages that cannot be repaired include:

file header page

database boot page

allocation bitmap pages (GAM, SGAM, PFS)

New DMV sys.dm_db_mirroring_auto_page_repair

Repair logged to SQL Server Error log

... Error: 824, Severity: 24, State: 2.

... SQL Server detected a logical consistency-based I/O error: incorrect checksum ...

... Database mirroring is attempting to repair physical page (4:4256) in database MyDB" by requesting a copy from the partner.

... Database mirroring successfully repaired physical page (4:4256) in database MyDB" by obtaining a copy from the partner.


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Log compression

DBM performance affected by network performance

Useful low-bandwidth scenarios

Outgoing log stream - principal to mirror compressed Requires CPU to compress/ decompress

Can be disabled

High-bandwidth network performance

Heavy duty, fully-logged maintenance operations when

mirroring is involved (e.g. index rebuild)


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Greatest improvement for low bandwidth networks

Log Compression Transactions/ second


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CPU usage

sometimes x2

Compressionoverhead

Increased

transaction

throughput

CPU Usage with Log Compression


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The business continuity problemCold/ Warm/ Hot Standby Solutions

Failover Clustering

Database Mirroring



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Maximize availability for

Scale outOffload primary data platform

Heavy reporting

Mobile/disconnected users

Autonomous business units that share data

Maximize availability of critical systems

Designed for failover

Fast, automatic

Zero data loss

Transactionally current

Masks planned and unplanned downtime

Replication

Failover Solutions


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Fault-Tolerant Publisher and Distributor

Subscribers

Distributor

PublisherFailover Solution + Replication


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Principal Server can be a Failover Cluster Failover to mirror will occur before failover within the

cluster

Principal will assume role of Mirror

Mirror can be a Failover Cluster as well

Principal Mirror

Failover Cluster Failover Cluster


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Attribute Failover Cluster Log Shipping Database Mirroring

Automatic failover Yes No Yes, in High Safety

Perceived downtime 30 secs + recovery n/a < 5 secs

Potential data loss Single copy of dataYes, most recenttransaction log

Yes, in someconfigurations

Masking storage

failureNo Yes Yes

Special hardware Must pass validation No No

ScopeSystem and user

databasesUser databases User databases

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