Enhancing General Replication Performance
You can enhance the general performance for all types of replication in your application and on your network by using the guidelines described in this topic.
Server and Network
- Set the minimum and maximum amount of memory allocated to Microsoft SQL Server Database Engine.
By default, the Database Engine changes its memory requirements dynamically based on available system resources. To avoid low memory availability during replication activities, use the min server memory option to set the minimum available memory. To avoid having the operating system page to disc for memory, you can also set a maximum amount of memory with the max server memory option. For more information, see Server Memory Options.
- Ensure proper allocation of database data files and log files. Use a separate disk drive for the transaction log for all databases involved in replication.
You can decrease the time it takes to write transactions by storing the log files on a disk drive different than the one used to store the database. You can mirror that drive, using a Redundant Array of Inexpensive Disks (RAID)-1, if you require fault tolerance. Use RAID 0 or 0+1 (depending on your need for fault tolerance) for other database files. This is a good practice regardless of whether or not replication is being used. For more information, see RAID Levels and SQL Server.
- Consider adding memory to servers used in replication, particularly the Distributor.
- Use multiprocessor computers.
Replication agents can take advantage of additional processors on the server. If you are running at high CPU usage, consider installing a faster CPU or multiple CPUs.
- Use a fast network.
The network can be a significant performance bottleneck, particularly for transactional replication. The propagation of changes to Subscribers can be significantly enhanced by using a fast network of 100 megabits per second (Mbps) or faster. If the network is slow, specify appropriate network settings and agent parameters. For more information, see A Slow Network Is Causing Problems.
Follow best practices for database design.
A replicated database generally benefits from the same performance optimizations as a non-replicated database. However, indexes should be used with caution at the Subscriber: the primary key column at the Subscriber should be indexed, but additional indexes can affect insert, update, and delete performance. For more information about optimizing databases, see Optimizing Databases.
Consider setting the READ_COMMITTED_SNAPSHOT database option.
To help reduce contention between user activity and replication agent activity, set this option for the publication and subscription databases:
ALTER DATABASE AdventureWorks SET READ_COMMITTED_SNAPSHOT ON
For more information, see ALTER DATABASE (Transact-SQL).
Be cautious with application logic in triggers.
Business logic in user-defined triggers at the Subscriber can slow down the replication of changes to the Subscriber:
- For transactional replication, it can be more efficient to include this logic in custom stored procedures used to apply the replicated commands. For more information, see Specifying How Changes Are Propagated for Transactional Articles.
- For merge replication, it can be more efficient to use business logic handlers. For more information, see Executing Business Logic During Merge Synchronization.
If you do use triggers at the Subscriber, see the following topics for more information: Controlling Constraints, Identities, and Triggers with NOT FOR REPLICATION and Considerations for Transactional Replication. If you use triggers to maintain referential integrity in tables published for merge replication, specify the processing order of tables to reduce the number of retries required for the Merge Agent. For more information, see Specifying the Processing Order of Merge Articles.
Limit the use of Large Object (LOB) data types.
LOBs require more storage space and processing than other column data types. Do not include these columns in articles unless necessary for your application. The data types text, ntext, and image are deprecated. If you do include LOBs, we recommend that you use the data types varchar(max), nvarchar(max), varbinary(max), respectively.
For transactional replication, consider using the Distribution Agent profile called Distribution Profile for OLEDB streaming. For more information, see Replication Agent Profiles.
- Publish only the data required.
Because replication is easy to set up, there is a tendency to publish more data than is actually required. This can consume additional resources within the distribution databases and snapshot files, and can lower the throughput for required data. Avoid publishing unnecessary tables and consider updating publications less frequently.
- Minimize conflicts through publication design and application behavior.
The following types of replication allow data to be changed at Subscribers: merge replication, transactional replication with updatable subscriptions, and peer-to-peer transactional replication. Merge replication and transactional replication with updatable subscriptions support data conflicts if a given row is updated at more than one node between synchronizations. Peer-to-peer replication does not support data conflicts; data changes must be partitioned. Regardless of the type of replication used, we recommend that you partition changes whenever possible, because this reduces the processing required for conflict detection and resolution.
Changes can be partitioned by publishing subsets of data to each Subscriber or by having an application direct changes for a given row to a given node:
- Merge replication supports publishing subsets of data using parameterized filters with a single publication. For more information, see Parameterized Row Filters.
- Transactional replication supports publishing subsets of data using static filters with multiple publications. For more information, see Filtering Published Data.
- Use row filters judiciously.
When a transactional publication includes one or more articles that use row filters, the Log Reader Agent must apply the filter to each row affected by an update to the table as it scans the transaction log. The throughput of the Log Reader Agent is therefore affected.
Similarly, merge replication must evaluate changed or deleted rows to determine which Subscribers should receive those rows. When row filters are employed to reduce the data required at a Subscriber, this processing is more complex and can be slower than when you publish all rows in a table. Consider carefully the tradeoff between reduced storage requirements at each Subscriber and the need for achieving maximum throughput. For more information on filtering, see Filtering Published Data.
- Use pull subscriptions when there are a large number of Subscribers.
The Distribution Agent and Merge Agent run at the Distributor for push subscriptions, and at Subscribers for pull subscriptions. Using pull subscriptions can improve performance by moving agent processing from the Distributor to Subscribers. For more information, see Subscribing to Publications.
- Consider reinitialization of the subscription if Subscribers are too far behind.
When large amounts of changes need to be sent to Subscribers, reinitializing them with a new snapshot might be faster than using replication to move the individual changes. For more information, see Reinitializing a Subscription.
For transactional replication, Replication Monitor displays on the Undistributed Commands tab information about: the number of transactions in the distribution database that have not yet been distributed to a Subscriber; and the estimated time for distributing these transactions. For more information, see How to: View Information and Perform Tasks for the Agents Associated With a Subscription (Replication Monitor).
- Run the Snapshot Agent only when necessary and at off-peak times.
The Snapshot Agent bulk copies data from the published table on the Publisher to a file in the snapshot folder on the Distributor. Generating a snapshot can be a resource intensive process and is best scheduled during off-peak times.
- Use a native mode snapshot unless a character mode snapshot is required.
Use the default native mode snapshot for all Subscribers except non-SQL Server Subscribers and Subscribers running SQL Server 2005 Compact Edition, which require a character mode snapshot.
- Use a single snapshot folder for a publication.
When specifying the publication properties related to snapshot location, you can choose to generate snapshot files to the default snapshot folder, to an alternate snapshot folder, or to both. Generating snapshot files in both locations requires additional disk space and more processing when the Snapshot Agent runs.
- Place the snapshot folder on a drive local to the Distributor that is not used to store database or log files.
The Snapshot Agent performs a sequential write of data to the snapshot folder. Placing the snapshot folder on a separate drive from any database or log files reduces contention among the disks and helps the snapshot process complete faster.
- When you create the subscription database at the Subscriber, consider specifying a recovery model of simple or bulk-logged. This allows minimal logging of the bulk inserts performed during the application of the snapshot at the Subscriber. After the snapshot has been applied to the subscription database, you can change to a different recovery model if necessary (replicated databases can use any of the recovery models). For more information about selecting a recovery model, see Overview of Restore and Recovery in SQL Server.
- Consider using the alternate snapshot folder and compressed snapshots on removable media for low-bandwidth networks.
Compressing snapshot files in the alternate snapshot folder can reduce snapshot disk storage requirements and make it easier to transfer snapshot files on removable media.
Compressed snapshots can, in some cases, improve the performance of transferring snapshot files across the network. However, compressing the snapshot requires additional processing by the Snapshot Agent when generating the snapshot files, and by the Distribution Agent or Merge Agent when applying the snapshot files. This may slow down snapshot generation and increase the time it takes to apply a snapshot in some cases. Additionally, compressed snapshots cannot be resumed if a network failure occurs; therefore they are not suitable for unreliable networks. Consider these tradeoffs carefully when using compressed snapshots across a network. For more information, see Alternate Snapshot Folder Locations and Compressed Snapshots.
- Consider initializing a subscription manually.
In some scenarios, such as those involving large initial datasets, it is preferable to initialize a subscription using a method other than a snapshot. For more information, see Initializing a Transactional Subscription Without a Snapshot and Initializing a Merge Subscription Without a Snapshot.
- Reduce the verbose levels of replication agents except during initial testing, monitoring, or debugging.
Reduce the –HistoryVerboseLevel parameter and the –OutputVerboseLevel parameter of the Distribution Agents or Merge Agents. This reduces the number of new rows inserted to track agent history and output. Instead, previous history messages with the same status are updated to the new history information. Increase the verbose levels for testing, monitoring, and debugging so that you have as much information about agent activity as possible.
- Use the –MaxBCPThreads parameter of the Snapshot Agent, Merge Agent, and Distribution Agent (the number of threads specified should not exceed the number of processors on the computer). This parameter specifies the number of bulk copy operations that can be performed in parallel when the snapshot is created and applied.
- Use the –UseInprocLoader parameter of the Distribution Agent and the Merge Agent (this parameter cannot be used if published tables include XML columns). This parameter causes the agent to use the BULK INSERT command when the snapshot is applied.
Agent parameters can be specified in agent profiles and on the command line. For more information, see:
- How to: Work with Replication Agent Profiles (SQL Server Management Studio)
- How to: View and Modify Replication Agent Command Prompt Parameters (SQL Server Management Studio)
- How to: Work with Replication Agent Profiles (Replication Transact-SQL Programming)
- Programming Replication Agent Executables.