Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached.

I was working on a SSRS report in SQL Server 2008 R2, report had So Many Datasets. When I was running the report I was getting the message "Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached." I tried to run the individual dataset's script in SSMS, it was working fine. I googled and Increase the Pool Size and Timeout duration in DataSource Connection String. Default Pool Size value is 100.

Data Source=ServerName;Initial Catalog= DatabaseName; Max Pool Size=500; Connection Timeout=1000;

This worked for me. For other applications like Asp.Net and others can use the connection string like:

ConnectionString="Data Source=ServerName;Initial Catalog= DatabaseName;Persist Security Info=True;User ID=SA;Password=UserPassword;Pooling='True'; Max Pool Size=500; Connection Timeout=1000;"

Reference: Govind Badkur(http://sqlserver20.blogspot.com)