1. Purpose, Pitfalls and Performance Implications
Linked Servers
Purpose, Pitfalls and Performance Implications
Linked Servers is an important tool in the DBA tool belt that, like most tools, can be super helpful or cause pain and anguish depending on how they are used. We are going to define what Linked Servers are, how they can be used, some tips on how best to use them, important performance implications, and some real-world query tuning examples.

2. About Ben Davis
Senior Consultant specializing in SQL Server 10 years working with SQL Server Background in infrastructure, virtualization, storage, industrial automation Experience primarily in manufacturing and financial services verticals Resides in Lafayette, IN with wife and three boys LinkedIn - Hobbies – home mechanic, track days, ketogenic (low-carb) lifestyle

3. What is a linked server?
A remote data source that SQL Server can access directly in queries
Can be another SQL instance or another OLE DB compliant product such as Oracle, IBM DB2, or Excel
SELECT * FROM [SQL2].[DB].[dbo].[table]
SQL Server acts as a client to another data source

4. Why would I use a linked server?
Joining local & remote data
Not feasible to bring data into local instance
Managed/owned by someone else
Primary use is by other applications, your use is minor/occasional
Change rate is too frequent to consider for ETL
Convenience
No need to handle joining in the app
Skillset present to write the join in T-SQL instead of thru app
Network not accessible by app server
Inability of SQL client application to connect to another data source type

5. How to setup
1. Add SQL Server linked server. 2. Add Azure linked server. 3. Add Oracle provider. C:\Users\benjamin.davis2\Downloads\ODAC122010Xcopy_x64\install.bat all c:\oracle odac

6. Best Practices Bring over as little data as possible
If repeated access to the same data, save data in a local temp table for quick access
Use should be uncommon, not for regular transactional use. If regular access needed, consider co-locating databases
Consider multiple app connections and joining data in the app

7. MS DTC
Microsoft Distributed Transaction Coordinator is required for transactions across instances
Need to enable a few things in the Component Services applet
Network DTC Access
Allow Remote Clients
Allow Inbound/Outbound
Mutual Authentication or No Authentication Required

8. MS DTC Configuration

9. XACT_ABORT
Will rollback entire distributed transaction if a run-time error occurs (like a PK constraint failure) when ON
If set to OFF (default setting), the individual statement fails but the remainder of statements in the transaction are committed
Must be ON for transactions against OLE DB providers, including SQL Server

10. Support for Mirroring, AGs
Until SQL 2016, distributed transactions were not supported for database mirroring or Availability Groups.
2016 allowed this, but only if databases were on different servers
2017 has full support for distributed transactions on Availability Groups no matter the scenario

11. Distributed Transaction Support on SQL Server 2016
DB1 primary
DB1 secondary
DB2 primary
DB2 secondary
WSFC Instance 1
WSFC Instance 2
WSFC Instance 3
WSFC Instance 4
SUPPORTED
DB1 primary
DB1 secondary
DB2 primary
DB2 secondary
WSFC Instance 1
WSFC Instance 2
WSFC Instance 3
NOT SUPPORTED

12. Tuning example
SQL job with linked server references was taking 9.5 hours every day to run.
The code had linked server calls and the called code had linked server calls back.
Left graph is the linked server source instance wait. Right is the target instance wait.
Wait type on source instance was a combination of CPU and OLEDB.
Wait type on the target instance was all OLEDB.

13. Diagram of query EXEC spLinked2017 SQL2017 SQL2016 EXEC spLinked2016
SELECT * FROM Customers…

14. Tuning example
Clustered index scan operation on local table sub-select taking 97% of plan time
No index on datetime column in WHERE clause – was doing Clustered Index Scan
Added non-clustered index on datetime column
Decreased job time from 9.5 to 1.5 hours

15. Tuning example
Next day overall instance wait time after this one change
But you think I was satisfied? Heck no! This was too much fun!

16. Tuning example
Step 2 – most tables in stored procedure were accessing remote tables.
How about creating a remote version of the stored procedure to call from local SQL Agent?
That way, most of the work is done on the instance with the majority of the data on it

17. Tuning example
Step 3 – grab what little remote data there is from the new remote stored procedure into a temp table as it is use repeatedly
Step 4 – Instead of a “NOT IN (SELECT column)”, often a LEFT JOIN between tables with “WHERE column IS NULL” is faster.

18. Tuning result
Implementing Steps 2-4 further decreased run time to 16 minutes

19.
Ben Davis
Senior Consultant
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