WSUS sync fails with HTTP 503 errors

During the SCCM 2012 Migration, as soon as we complete, we probably get this error where number of SCCM client is high.

Error on SMS_WSUS_SYNC_MANAGER component

Message ID: 6703
WSUS Synchronization failed.
Message: The request failed with HTTP status 503: Service Unavailable.
Source: Microsoft.UpdateServices.Administration.AdminProxy.CreateUpdateServer.

When you attempt to open Update Services on the WSUS server you receive the following error:

error: Connection Error
An error occurred trying to connect to the WSUS server. This error can happen for a number of reasons. Please contact your network administrator if the problem persists. Click the Reset Server Node to connect to the server again.

In wsyncmgr.log you will get the below error,

Sync failed: The request failed with HTTP status 503: Service Unavailable.


In this situation, the most likely cause is that the WsusPool Application Pool in IIS is in a stopped state, as shown below.

Also, the Private Memory Limit (KB) for the Application Pool is probably set to the default value of 1843200 KB.

If you encounter this problem, increase the Private Memory Limit to 4GB (4000000 KB) and restart the Application Pool. To increase the Private Memory Limit, select the WsusPool Application Pool and click Advanced Settings under Edit Application Pool. Then set the Private Memory Limit to 4GB (4000000 KB).

After the Application Pool has been restarted, monitor the SMS_WSUS_SYNC_MANAGER component status, wcm.log and wsyncmgr.log for failures. Please note that it may be necessary to increase the Private Memory Limit to 8GB (8000000 KB) or higher depending on the environment.