Understand how AlwaysOn Active Secondaries allow leveraging
HA/DR hardware
Read Workloads
Backups
Offloading read workloads
Offloading backups
SQL Server 2012 HA/DR technologies
Failover Cluster Instances
for servers
Availability Groups
for groups of databases
Pre-existent
New
Server failover
Multi-database Failover
Shared Storage (SAN / SMB)
Direct attached storage
Failover takes minutes
Failover takes seconds
Multi-Node Clustering
Multiple Secondaries
Passive secondary nodes
Active
ActiveSecondaries
Secondaries
Active Secondaries allow using hardware for more than HA/DR:
Offload work from primary
Read workloads
Backups
Database Mirroring
Run on primary
Impacts workload
Transactional Replication
Run on a subscriber
Pros:
Run on mirror w/snapshots
Data Staleness
Additional management
No failover of read workloads
Cons:
Large number of subscribers
Targeted indexes
Separate solution to configure/manage
Replication latency
No failover of read workloads
SQL Server
Active
Secondary
Primary
SQL Server
Log
Synchronization
DB1
Reports
DB1
DB2
DB2
DBs in active secondaries are readable
Committed transactions visible eventually (depends on log
synchronization latency)
Same data center:
Different data center:
<1s
1-5s
SQL Server
DB1
Up to 4 secondaries
Sync or async
DB2
Primary
SQL Server
Secondary
Primary
Secondary
Primary
SQL Server
Log
Synchronization
DB1
DB2
Reports
DB1
DB2
Reports
NO
ALL
READ_ONLY
Don’t allow connections
Allow all connections
Only allow connections specifying READ_ONLY intent
ALTER AVAILABILITY GROUP ag_name
MODIFY REPLICA ON 'server_name'
WITH (
SECONDARY_ROLE (
ALLOW_CONNECTIONS = { NO | ALL | READ_ONLY } ) )
Clients can connect to readable secondary without changes
Set ALLOW_CONNECTIONS = ALL
If possible, use Application Intent
New connection string property
Specifies workload type: READ_ONLY / READ_WRITE
Opens new capabilities
Disallow connections
Automatically route READ_ONLY connections to an active secondary
Configure Routing URL for each secondary
Endpoint for read-only connections
ALTER AVAILABILITY GROUP ag_name
MODIFY REPLICA ON 'server_name'
WITH (
SECONDARY_ROLE (
READ_ONLY_ROUTING_URL = ‘TCP://system-address:port’ ) )
ALTER AVAILABILITY GROUP ag_name
MODIFY REPLICA ON 'server_name'
WITH (
PRIMARY_ROLE (
READ_ONLY_ROUTING_LIST = {'server_name' [,..n]} ) )
Mitigations:
Use Resource Governor
No workload on auto failover target
Contention between Redo and read workload avoided internally
Workload executed on Snapshot Isolation
Locking hints ignored
Redo thread never a deadlock victim
DDL Redo can still be blocked by read workload
DDL is infrequent - if possible run when low activity
XEvent lock_redo_blocked
Query optimization relies on statistics
Created by indexes and read workloads
Statistics created on primary are redone on secondary
But, read workloads at secondaries are different from
primary workloads
Auto-create statistics on secondary
Store on TempDB
sys.stats: is_temporary=‘true’
Use most recent statistics
Remove on failover, restart, or DROP STATISTICS
Database Mirroring
R/W workload
Failed
Backups
Backups
Primary
Only on primary
Impacts primary workload
Fails after failover
Secondary
R/W workload
Backups
Active
Secondary
Backups
Backups
Primary
Active
Secondary
On primary or any secondary
No need to configure secondaries as readable
Backup on sync or async secondaries
Full Backup (Copy-Only)
Log Backup
Backup automation
Full Backup (Copy-Only) and Log Backup on secondary
If Differential Backups are needed:
Full Backup and Differential Backup on primary
Log Backup on secondary
Role Preference
Primary Only
Secondary Only
Secondary If Possible
None
Replica Priority
0 (never) ... 100 (highest)
Filter out replicas
not online
don’t meet role preference
Select highest-priority replica
Break ties with replica name
Not enforced by BACKUP command
Implemented as a system function
sys.fn_hadr_backup_is_preferred_replica ('dbname')
Used by
Maintenance Plan Wizard
Log Shipping Wizard
Store backups centrally
Easier to track
Use Recovery Advisor
Easier to restore to point in time
are not
Active Secondaries make AlwaysOn cost-effective
Use hardware for more than HA/DR: Offload work from primary
Read workloads
Without client changes
With Application Intent: Read-only Routing (provides HA)
Backups
Full Backup (copy-only) and Log Backup
Automation supported
Windows Azure support is coming
[email protected]
social.msdn.microsoft.com/Forums/en-gb/sqlhadr/
SQL Server 2012
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