sql-server - 为什么SqlConnection.Open()可能会出现超时？

Question

SqlConnection.Open()发生超时的情况有哪些？

在 AppProcess 回收后 10 秒，我们的一台 IIS 机器上出现以下异常:

Type : System.Data.SqlClient.SqlException, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
    Message : Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.
    Source : .Net SqlClient Data Provider
    Help link : 
    Errors : System.Data.SqlClient.SqlErrorCollection
    Class : 11
    LineNumber : 0
    Number : -2
    Procedure : 
    Server : XXX
    State : 0
    ErrorCode : -2146232060
    Data : System.Collections.ListDictionaryInternal
    TargetSite : Void OnError(System.Data.SqlClient.SqlException, Boolean)
    Stack Trace :    at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection)
       at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj)
       at System.Data.SqlClient.TdsParserStateObject.ReadSniError(TdsParserStateObject stateObj, UInt32 error)
       at System.Data.SqlClient.TdsParserStateObject.ReadSni(DbAsyncResult asyncResult, TdsParserStateObject stateObj)
       at System.Data.SqlClient.TdsParserStateObject.ReadNetworkPacket()
       at System.Data.SqlClient.TdsParser.ConsumePreLoginHandshake(Boolean encrypt, Boolean trustServerCert, Boolean& marsCapable)
       at System.Data.SqlClient.TdsParser.Connect(ServerInfo serverInfo, SqlInternalConnectionTds connHandler, Boolean ignoreSniOpenTimeout, Int64 timerExpire, Boolean encrypt, Boolean trustServerCert, Boolean integratedSecurity, SqlConnection owningObject)
       at System.Data.SqlClient.SqlInternalConnectionTds.AttemptOneLogin(ServerInfo serverInfo, String newPassword, Boolean ignoreSniOpenTimeout, Int64 timerExpire, SqlConnection owningObject)
       at System.Data.SqlClient.SqlInternalConnectionTds.LoginWithFailover(Boolean useFailoverHost, String primaryHost, String failoverHost, String newPassword, Boolean redirectedUserInstance, SqlConnection owningObject, SqlConnectionString connectionOptions, Int64 timerStart)
       at System.Data.SqlClient.SqlInternalConnectionTds.OpenLoginEnlist(SqlConnection owningObject, SqlConnectionString connectionOptions, String newPassword, Boolean redirectedUserInstance)
       at System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, Object providerInfo, String newPassword, SqlConnection owningObject, Boolean redirectedUserInstance)
       at System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection)
       at System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnection owningConnection, DbConnectionPool pool, DbConnectionOptions options)
       at System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject)
       at System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject)
       at System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject)
       at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
       at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
       at System.Data.SqlClient.SqlConnection.Open()
       at NHibernate.Connection.DriverConnectionProvider.GetConnection()
       at NHibernate.Impl.SessionFactoryImpl.OpenConnection()

根据MSDN Number 属性包含 SQL 引擎错误。但我无法在 master.dbo.sysmessages 中找到错误 -2。尝试打开连接的操作之间的间隔约为 3.5 秒。连接超时尚未显式更改。

2秒后，同一个线程出现了类似的异常，但在不同的地方。调用堆栈要短得多:

    Type : System.Data.SqlClient.SqlException, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
Message : Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.
Source : .Net SqlClient Data Provider
Help link : 
Errors : System.Data.SqlClient.SqlErrorCollection
Class : 11
LineNumber : 0
Number : -2
Procedure : 
Server : XXX
State : 0
ErrorCode : -2146232060
Data : System.Collections.ListDictionaryInternal
TargetSite : Void OnError(System.Data.SqlClient.SqlException, Boolean)
Stack Trace :    at System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject)
   at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
   at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
   at System.Data.SqlClient.SqlConnection.Open()
   at NHibernate.Connection.DriverConnectionProvider.GetConnection()
   at NHibernate.Impl.SessionFactoryImpl.OpenConnection()

尝试与数据库通信的操作与错误之间的间隔小于秒。

问题:1.如何重现此类错误？2. 为什么第二种情况下调用堆栈更短？

Answer 1

问题似乎是由镜像引起的。同样的情况描述here