SQL Server 2016 Availability Group Automatic Seeding

By:Simon Liew || Related Tips:More > AlwaysOn Availability

Problem

Automatic Seeding for an Availability Group (AG) is a new addition to the way databases can be added into an AG in SQL Server 2016. This tip will give an introduction on Automatic Seeding and compare performance adding databases into an AG between Automatic Seeding and adding a database to an AG using backup and restore.

Solution Since the introduction of AG, adding a database into an AG involves a databasebackup andrestore operation quite similar to configuring database mirroring. As part of the backup and restore process, the database backup needs to reside on a (shared) folder accessible to all SQL replicas for the restore operation.

Starting with SQL Server 2016, Automatic Seeding is introduced as a new way to add databases into an AG. Automatic Seeding reads the database files directly and streams the bytes to the secondary using the database mirroring endpoints without requiring an explicit backup and restore of the database during the process. This also means the I/O overhead involved with backup and restore operation to a physical file can now be avoided.

During Automatic Seeding, the Dynamic Management View (DMV) sys.dm_exec_requests exposes some information such as the percent_complete of the streaming. These are background processes which means it is scheduled internally by SQL Server. Transaction log truncation will be blocked during the Automatic Seeding activity. So, this is an important consideration if workloads are allowed on the database prior to the completion of the seeding process.

SELECT
r.session_id, r.status, r.command, r.wait_type
, r.percent_complete, r.estimated_compleation_time
FROM sys.dm_exec_requests r JOIN sys.dm_exec_sessions s
ON r.session_id = s.sessiond_id
WHERE r.session_id <> @@SPID
AND s.is_user_process = 0
AND r.command like 'VDI%'

Automatic Seeding is a replica level setting and applies to all the databases in the AG. A documented trace flag 9567 can be turned on for the primary SQL instance during the automatic seeding process to enable compression of the data stream. This trace flag can significantly reduce the transfer time, but at the same time increases the load on the server CPU utilization.

There are two DMVs to view information on Automatic Seeding activity.

DMV

Description

sys.dm_hadr_physical_seeding_stats Provides statistics during the automatic seeding process sys.dm_hadr_automatic_seeding Provides status on automatic seeding process Test Configuration

Below is some high-level information of the testing environment configuration. The database used in this test is not TDE-enabled.

Database size 60 GB Database space used 47 GB Transaction log size 5 GB Backup size without compression 47 GB Backup size with compression 400 MB AG nodes 2 Network links 10.0 Gpbs Storage SAN storage (rotating media) on RAID-10

The same database is restored each time prior to adding the database into the AG. The AG endpoint and AlwaysOn_health XE are created prior to starting the test.

Test Results Test Test Scenario Duration 1 Automatic Seeding without TF9567 12 mins 05 secs 2 Backup and restore without backup compression 13 mins 30 secs 3 Automatic Seeding with TF9567 2 mins 56 secs 4 Backup and restore with backup compression 5 mins 12 secs Summary

The backup and restore method to add a database into an AG is still applicable and fully supported. The similar conditions to add databases into an AG are still applicable to both Automatic Seeding or backup / restore such as the database must be in the FULL recovery model and have a current full backup.

The test database only contains a single table containing the same values, hence the database backup compression can compress to an impressive backup size of only 400 MB. Disregarding this fact, adding databases into an AG using Automatic Seeding with TF9567 outperformed the equivalent of the backup and restore method with compression.

Tests Configuration and Steps

Below are the steps I used for the different tests.

Test 1 Automatic Seeding without TF9567 Restore the test database on the primary SQL instance. The script to be executed on the primary SQL instance below can be easily generated using the New Availability Group Wizard. To use Automatic Seeding, the script only needs to include an additional argument SEEDING_MODE = AUTOMATIC.

Script to be executed on primary SQL instance.

CREATE AVAILABILITY GROUP [agname]
WITH (AUTOMATED_BACKUP_PREFERENCE = PRIMARY,
DB_FAILOVER = OFF,
DTC_SUPPORT = NONE)
FOR DATABASE [dbname]
REPLICA ON N'<primary_server_instance>' WITH (ENDPOINT_URL = N'TCP://<primary_server>.<fully_qualified_domain_name>:5022'
, FAILOVER_MODE = MANUAL
, AVAILABILITY_MODE = SYNCHRONOUS_COMMIT
, BACKUP_PRIORITY = 50
, SECONDARY_ROLE(ALLOW_CONNECTIONS = NO)
, SEEDING_MODE = AUTOMATIC),
N'<secondary_server_instance>' WITH (ENDPOINT_URL = N'TCP://<secondary_server>.<fully_qualified_domain_name>:5022'
, FAILOVER_MODE = MANUAL
, AVAILABILITY_MODE = SYNCHRONOUS_COMMIT
, BACKUP_PRIORITY = 50
, SECONDARY_ROLE(ALLOW_CONNECTIONS = NO)
, SEEDING_MODE = AUTOMATIC);
GO On the secondary SQL instance, an additional line of code is required to grant the AG permission to create the database. ALTER AVAILABILITY GROUP [agname] JOIN;
GO
ALTER AVAILABILITY GROUP [agname] GRANT CREATE ANY DATABASE;
GO When automatic seeding is enabled for an AG, new databases will automatically get added to the secondary. If you do not want this behavior, the script below will disable the automatic seeding on the primary SQL replica. ALTER AVAILABILITY GROUP [agname ]
MODIFY REPLICA ON ' <primary_server_instance>'
WITH (SEEDING_MODE = MANUAL)
GO
Test 2 - Backup and restore without backup compression Restore the test database on the primary SQL instance. The script below is generated using the New Availability Group Wizard and then executed in a SQLCMD mode to allow continuation of the backup and restore operation immediately after each step. --- YOU MUST EXECUTE THE FOLLOWING SCRIPT IN SQLCMD MODE.
:Connect <primary_server_instance>
USE [master]
GO
CREATE AVAILABILITY GROUP [agname]
WITH (AUTOMATED_BACKUP_PREFERENCE = PRIMARY,
DB_FAILOVER =