Merge pull request #100427 from MashaMSFT/patch-76

added notes about initial seeding phase

Author: PRMerger20

articles/sql-database/sql-database-auto-failover-group.md

@@ -67,6 +67,13 @@ To achieve real business continuity, adding database redundancy between datacent
 
   You can put all or several databases within an elastic pool into the same failover group. If the primary database is in an elastic pool, the secondary is automatically created in the elastic pool with the same name (secondary pool). You must ensure that the secondary server contains an elastic pool with the same exact name and enough free capacity to host the secondary databases that will be created by the failover group. If you add a database in the pool that already has a secondary database in the secondary pool, that geo-replication link is inherited by the group. When you add a database that already has a secondary database in a server that is not part of the failover group, a new secondary is created in the secondary pool.
 
+- **Initial Seeding** 
+
+  When adding databases, elastic pools, or managed instances to a failover group, there is an initial seeding phase before data replication starts. The initial seeding phase is the longest and most expensive operation. Once initial seeding completes, data is synchronized, and then only subsequent data changes are replicated. The time it takes for the initial seed to complete depends on the size of your data, number of replicated databases, and the speed of the link between the entities in the failover group. Under normal circumstances, typical seeding speed is 50-500 GB an hour for a single database or elastic pool, and 18-35 GB an hour for a managed instance. Seeding is performed for all databases in parallel. You can use the stated seeding speed, along with the number of databases and the total size of data to estimate how long the initial seeding phase will take before data replication starts.
+
+  For managed instances, the speed of the Express Route link between the two instances also needs to be considered when estimating the time of the initial seeding phase. If the speed of the link between the two instances is slower than what is necessary, the time to seed is likely be notably impacted. You can use the stated seeding speed, number of databases, total size of data, and the link speed to estimate how long the initial seeding phase will take before data replication starts. For example, for a single 100 GB database, the initial seed phase would take anywhere from 2.8 - 5.5 hours if the link is capable of pushing 35 GB per hour. If the link can only transfer 10 GB per hour, then seeding a 100 GB database will take about 10 hours. If there are multiple databases to replicate, seeding will be executed in parallel, and, when combined with a slow link speed, the initial seeding phase may take considerably longer, especially if the parallel seeding of data from all databases exceeds the available link bandwidth. If the network bandwidth between  two instances is limited and you are adding multiple managed instances to a failover group, consider adding multiple managed instances to the failover group sequentially, one by one.
+
+  
 - **DNS zone**
 
   A unique ID that is automatically generated when a new instance is created. A multi-domain (SAN) certificate for this instance is provisioned to authenticate the client connections to any instance in the same DNS zone. The two managed instances in the same failover group must share the DNS zone.