Merge pull request #14736 from MikeRayMSFT/20200427-DAC

Update DAC instructions

Author: PMEds28

docs/database-engine/configure-windows/diagnostic-connection-for-database-administrators.md

@@ -36,46 +36,54 @@ ms.author: mikeray
 
  Only members of the [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] sysadmin role can connect using the DAC.  
 
- The DAC is available and supported through the **sqlcmd** command-prompt utility using a special administrator switch (**-A**). For more information about using **sqlcmd**, see [Use sqlcmd with Scripting Variables](../../relational-databases/scripting/sqlcmd-use-with-scripting-variables.md). You can also connect prefixing **admin:** to the instance name in the format **sqlcmd -S admin:<*instance_name*>**. You can also initiate a DAC from a [!INCLUDE[ssManStudioFull](../../includes/ssmanstudiofull-md.md)] Query Editor by connecting to **admin:\<*instance_name*>**.  
-  
+ The DAC is available and supported through the `sqlcmd` command-prompt utility using a special administrator switch (`-A`). For more information about using `sqlcmd`, see [Use sqlcmd with Scripting Variables](../../relational-databases/scripting/sqlcmd-use-with-scripting-variables.md). You can also connect prefixing `admin:` to the instance name in the format `sqlcmd -S admin:<*instance_name*>`. You can also initiate a DAC from a [!INCLUDE[ssManStudioFull](../../includes/ssmanstudiofull-md.md)] Query Editor by connecting to `admin:\<*instance_name*>`.
+
+> [!Note]  
+> To establish a DAC from [!INCLUDE[ssManStudioFull](../../includes/ssmanstudiofull-md.md)]
+> - Disconnect all connections to the related SQL Server instance, including the Object Explorer and all open query windows.
+> - From the menu select **File** > **New** > **Database Engine Query**
+> - From the connection dialog box in the Server Name field, enter `admin:<server_name>` if using the default instance or `admin:<server_name>\<instance_name>` if using a named instance.
+
 ## Restrictions  
  Because the DAC exists solely for diagnosing server problems in rare circumstances, there are some restrictions on the connection:  
 
--   To guarantee that there are resources available for the connection, only one DAC is allowed per instance of [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)]. If a DAC connection is already active, any new request to connect through the DAC is denied with error 17810.  
-  
--   To conserve resources, [!INCLUDE[ssExpress](../../includes/ssexpress-md.md)] does not listen on the DAC port unless started with a trace flag 7806.  
-  
--   The DAC initially attempts to connect to the default database associated with the login. After it is successfully connected, you can connect to the master database. If the default database is offline or otherwise not available, the connection will return error 4060. However, it will succeed if you override the default database to connect to the master database instead using the following command:  
+- To guarantee that there are resources available for the connection, only one DAC is allowed per instance of [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)]. If a DAC connection is already active, any new request to connect through the DAC is denied with error 17810.  
 
-     **sqlcmd -A -d master**  
+- To conserve resources, [!INCLUDE[ssExpress](../../includes/ssexpress-md.md)] does not listen on the DAC port unless started with a trace flag 7806.  
 
-     We recommend that you connect to the master database with the DAC because master is guaranteed to be available if the instance of the [!INCLUDE[ssDE](../../includes/ssde-md.md)] is started.  
-  
--   [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] prohibits running parallel queries or commands with the DAC. For example, error 3637 is generated if you execute either of the following statements with the DAC:  
+- The DAC initially attempts to connect to the default database associated with the login. After it is successfully connected, you can connect to the master database. If the default database is offline or otherwise not available, the connection will return error 4060. However, it will succeed if you override the default database to connect to the master database instead using the following command:  
+
+   ```powershell
+   sqlcmd -A -d master 
+   ```
+
+   We recommend that you connect to the master database with the DAC because master is guaranteed to be available if the instance of the [!INCLUDE[ssDE](../../includes/ssde-md.md)] is started.  
 
-    -   RESTORE  
+- [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] prohibits running parallel queries or commands with the DAC. For example, error 3637 is generated if you execute either of the following statements with the DAC:  
 
-    -   BACKUP  
+  - `RESTORE...`
 
--   Only limited resources are guaranteed to be available with the DAC. Do not use the DAC to run resource-intensive queries (for example. a complex join on large table) or queries that may block. This helps prevent the DAC from compounding any existing server problems. To avoid potential blocking scenarios, if you have to run queries that may block, run the query under snapshot-based isolation levels if possible; otherwise, set the transaction isolation level to READ UNCOMMITTED and set the LOCK_TIMEOUT value to a short value such as 2000 milliseconds, or both. This will prevent the DAC session from getting blocked. However, depending on the state that the [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] is in, the DAC session might get blocked on a latch. You might be able to terminate the DAC session using CTRL-C but it is not guaranteed. In that case, your only option may be to restart [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)].  
+  - `BACKUP...`
+
+- Only limited resources are guaranteed to be available with the DAC. Do not use the DAC to run resource-intensive queries (for example. a complex join on large table) or queries that may block. This helps prevent the DAC from compounding any existing server problems. To avoid potential blocking scenarios, if you have to run queries that may block, run the query under snapshot-based isolation levels if possible; otherwise, set the transaction isolation level to READ UNCOMMITTED and set the LOCK_TIMEOUT value to a short value such as 2000 milliseconds, or both. This will prevent the DAC session from getting blocked. However, depending on the state that the [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] is in, the DAC session might get blocked on a latch. You might be able to terminate the DAC session using CTRL-C but it is not guaranteed. In that case, your only option may be to restart [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)].  
 
--   To guarantee connectivity and troubleshooting with the DAC, [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] reserves limited resources to process commands run on the DAC. These resources are typically only enough for simple diagnostic and troubleshooting functions, such as those listed below.  
+- To guarantee connectivity and troubleshooting with the DAC, [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] reserves limited resources to process commands run on the DAC. These resources are typically only enough for simple diagnostic and troubleshooting functions, such as those listed below.  
 
  Although you can theoretically run any [!INCLUDE[tsql](../../includes/tsql-md.md)] statement that does not have to execute in parallel on the DAC, we strongly recommend that you restrict usage to the following diagnostic and troubleshooting commands:  
 
--   Querying dynamic management views for basic diagnostics such as [sys.dm_tran_locks](../../relational-databases/system-dynamic-management-views/sys-dm-tran-locks-transact-sql.md) for the locking status, [sys.dm_os_memory_cache_counters](../../relational-databases/system-dynamic-management-views/sys-dm-os-memory-cache-counters-transact-sql.md) to check the health of caches, and [sys.dm_exec_requests](../../relational-databases/system-dynamic-management-views/sys-dm-exec-requests-transact-sql.md) and [sys.dm_exec_sessions](../../relational-databases/system-dynamic-management-views/sys-dm-exec-sessions-transact-sql.md) for active sessions and requests. Avoid dynamic management views that are resource intensive (for example, [sys.dm_tran_version_store](../../relational-databases/system-dynamic-management-views/sys-dm-tran-version-store-transact-sql.md) scans the full version store and can cause extensive I/O) or that use complex joins. For information about performance implications, see the documentation for the specific [dynamic management view](../../relational-databases/system-dynamic-management-views/system-dynamic-management-views.md).  
+- Querying dynamic management views for basic diagnostics such as [sys.dm_tran_locks](../../relational-databases/system-dynamic-management-views/sys-dm-tran-locks-transact-sql.md) for the locking status, [sys.dm_os_memory_cache_counters](../../relational-databases/system-dynamic-management-views/sys-dm-os-memory-cache-counters-transact-sql.md) to check the health of caches, and [sys.dm_exec_requests](../../relational-databases/system-dynamic-management-views/sys-dm-exec-requests-transact-sql.md) and [sys.dm_exec_sessions](../../relational-databases/system-dynamic-management-views/sys-dm-exec-sessions-transact-sql.md) for active sessions and requests. Avoid dynamic management views that are resource intensive (for example, [sys.dm_tran_version_store](../../relational-databases/system-dynamic-management-views/sys-dm-tran-version-store-transact-sql.md) scans the full version store and can cause extensive I/O) or that use complex joins. For information about performance implications, see the documentation for the specific [dynamic management view](../../relational-databases/system-dynamic-management-views/system-dynamic-management-views.md).  
 
--   Querying catalog views.  
+- Querying catalog views.  
 
--   Basic DBCC commands such as [DBCC FREEPROCCACHE](../..//t-sql/database-console-commands/dbcc-freeproccache-transact-sql.md), [DBCC FREESYSTEMCACHE](../../t-sql/database-console-commands/dbcc-freesystemcache-transact-sql.md), [DBCC DROPCLEANBUFFERS](../../t-sql/database-console-commands/dbcc-dropcleanbuffers-transact-sql.md), and [DBCC SQLPERF](../../t-sql/database-console-commands/dbcc-sqlperf-transact-sql.md). Do not run resource-intensive commands such as [DBCC CHECKDB](../../t-sql/database-console-commands/dbcc-checkdb-transact-sql.md), [DBCC DBREINDEX](../../t-sql/database-console-commands/dbcc-dbreindex-transact-sql.md), or [DBCC SHRINKDATABASE](../../t-sql/database-console-commands/dbcc-shrinkdatabase-transact-sql.md).  
+- Basic DBCC commands such as [DBCC FREEPROCCACHE](../..//t-sql/database-console-commands/dbcc-freeproccache-transact-sql.md), [DBCC FREESYSTEMCACHE](../../t-sql/database-console-commands/dbcc-freesystemcache-transact-sql.md), [DBCC DROPCLEANBUFFERS](../../t-sql/database-console-commands/dbcc-dropcleanbuffers-transact-sql.md), and [DBCC SQLPERF](../../t-sql/database-console-commands/dbcc-sqlperf-transact-sql.md). Do not run resource-intensive commands such as [DBCC CHECKDB](../../t-sql/database-console-commands/dbcc-checkdb-transact-sql.md), [DBCC DBREINDEX](../../t-sql/database-console-commands/dbcc-dbreindex-transact-sql.md), or [DBCC SHRINKDATABASE](../../t-sql/database-console-commands/dbcc-shrinkdatabase-transact-sql.md).  
 
--   [!INCLUDE[tsql](../../includes/tsql-md.md)] KILL*\<spid>* command. Depending on the state of [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)], the KILL command might not always succeed; then the only option may be to restart [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)]. The following are some general guidelines:  
+- [!INCLUDE[tsql](../../includes/tsql-md.md)] KILL*\<spid>* command. Depending on the state of [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)], the KILL command might not always succeed; then the only option may be to restart [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)]. The following are some general guidelines:  
 
-    -   Verify that the SPID was actually killed by querying `SELECT * FROM sys.dm_exec_sessions WHERE session_id = <spid>`. If it returns no rows, it means the session was killed.  
+    - Verify that the SPID was actually killed by querying `SELECT * FROM sys.dm_exec_sessions WHERE session_id = <spid>`. If it returns no rows, it means the session was killed.  
 
-    -   If the session is still there, verify whether there are tasks assigned to this session by running the query `SELECT * FROM sys.dm_os_tasks WHERE session_id = <spid>`. If you see the task there, most likely your session is currently being killed. Note that this may take considerable amount of time and may not succeed at all.  
+    - If the session is still there, verify whether there are tasks assigned to this session by running the query `SELECT * FROM sys.dm_os_tasks WHERE session_id = <spid>`. If you see the task there, most likely your session is currently being killed. Note that this may take considerable amount of time and may not succeed at all.  
 
-    -   If there are no tasks in the sys.dm_os_tasks associated with this session, but the session remains in sys.dm_exec_sessions after executing the KILL command, it means that you do not have a worker available. Select one of the currently running tasks (a task listed in the sys.dm_os_tasks view with a `sessions_id <> NULL`), and kill the session associated with it to free up the worker. Note that it may not be enough to kill a single session: you may have to kill multiple ones.  
+    - If there are no tasks in the sys.dm_os_tasks associated with this session, but the session remains in sys.dm_exec_sessions after executing the KILL command, it means that you do not have a worker available. Select one of the currently running tasks (a task listed in the sys.dm_os_tasks view with a `sessions_id <> NULL`), and kill the session associated with it to free up the worker. Note that it may not be enough to kill a single session: you may have to kill multiple ones.  
 
 ## DAC Port  
  [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] listens for the DAC on TCP port 1434 if available or a TCP port dynamically assigned upon [!INCLUDE[ssDE](../../includes/ssde-md.md)] startup. The [error log](../../relational-databases/performance/view-the-sql-server-error-log-sql-server-management-studio.md) contains the port number the DAC is listening on. By default the DAC listener accepts connection on only the local port. For a code sample that activates remote administration connections, see [remote admin connections Server Configuration Option](../../database-engine/configure-windows/remote-admin-connections-server-configuration-option.md).