昨天网上一网友说，由于他同事误将“max server memory”设置为10M后，SQL Server数据库登录不了，当时我简单测试了一下，今天有空就顺手将整个过程整理一下，记录在此。
在SSMS的UI界面设置“max server memory”，即使你设置为10M大小，但是它会“悄悄”默认修改为128M，你用Profile跟踪或者设置后会发现，它偷偷“修改”了你的设置值（改为了128M），
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EXEC sys.sp_configure N'max server memory (MB)', N'128'
GO
RECONFIGURE WITH OVERRIDE
GO
Configuration option 'max server memory (MB)' changed from 4096 to 128. Run the RECONFIGURE statement to install.
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如果你没有注意这些细节，或者不信这个事情，那么也可以用脚本测试一下，如下所示，它提示你这个值（10M）不是一个有效值。
当你对“max server memory”做了错误设置后，那么基本上，任何查询或连接都会出现类似下面这样的错误：
An exception occurred while executing a Transact-SQL statement or batch. (Microsoft.SqlServer.ConnectionInfo)
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There is insufficient system memory in resource pool 'internal' to run this query. (Microsoft SQL Server, Error: 701)
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ADDITIONAL INFORMATION:
A connection was successfully established with the server, but then an error occurred during the pre-login handshake. (provider: TCP Provider, error: 0 - 远程主机强迫关闭了一个现有的连接。) (Microsoft SQL Server, Error: 10054)
For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft%20SQL%20Server&EvtSrc=MSSQLServer&EvtID=10054&LinkId=20476
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远程主机强迫关闭了一个现有的连接。
你检查数据库的错误日志，就会发现有很多额外信息，摘抄部分如下：
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2019-12-24 10:15:32.84 spid53 There is insufficient system memory in resource pool 'internal' to run this query.
2019-12-24 10:15:52.88 spid53 Error: 18056, Severity: 20, State: 29. (Params:). The error is printed in terse mode because there was error during formatting. Tracing, ETW, notifications etc are skipped.
2019-12-24 10:15:55.89 Server Error: 17300, Severity: 16, State: 1. (Params:). The error is printed in terse mode because there was error during formatting. Tracing, ETW, notifications etc are skipped.
2019-12-24 10:16:12.70 Server Failed allocate pages: FAIL_PAGE_ALLOCATION 1
2019-12-24 10:16:12.70 Server
Process/System Counts Value
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Available Physical Memory 6614454272
Available Virtual Memory 140726213148672
Available Paging File 7776440320
Working Set 95432704
Percent of Committed Memory in WS 100
Page Faults 57030
System physical memory high 1
System physical memory low 0
Process physical memory low 1
Process virtual memory low 0
2019-12-24 10:16:12.70 Server
Memory Manager KB
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VM Reserved 10652776
VM Committed 57972
Locked Pages Allocated 86472
Large Pages Allocated 0
Emergency Memory 1024
Emergency Memory In Use 16
Target Committed 131072
Current Committed 144448
Pages Allocated 84176
Pages Reserved 0
Pages Free 0
Pages In Use 144432
Page Alloc Potential -19912
NUMA Growth Phase 2
Last OOM Factor 1
Last OS Error 0
2019-12-24 10:16:12.70 Server
Memory node Id = 0 KB
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VM Reserved 10652712
VM Committed 57952
Locked Pages Allocated 86472
Pages Allocated 84176
Pages Free 0
Target Committed 131048
Current Committed 144424
Foreign Committed 0
Away Committed 0
Taken Away Committed 0
2019-12-24 10:16:12.70 Server
Memory node Id = 64 KB
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VM Reserved 0
VM Committed 20
Locked Pages Allocated 0
2019-12-24 10:16:12.70 Server
MEMORYCLERK_SQLGENERAL (node 0) KB
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要解决这个问题，你需要关闭数据库服务， 然后以单用户模式+最小配置启动数据库实例，然后去修改max server memory参数。 关闭数据库过程中如果遇到一些问题，可以通过重启服务器解决问题（这个要根据具体实际情况决定，有时候不会遇到问题，有时候会遇到一些问题，例如net stop mssqlserver命令卡住，出现service_state[MSSQLSERVER]): Stop pending）
注意：如果以单用户模式启动，然后以sqlcmd去连接数据库，就会出现下面错误，所以必须以单用户模式+最小配置启动数据库实例
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EXEC sys.sp_configure 'max server memory (MB)',4096; #根据实际情况设置内存大小。
RECONFIGURE
GO
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然后重启SQL Server实例，问题就解决了。 当然你也可以还原master库的备份到其它测试数据库，然后用还原后master数据库的相关文件替换当前数据库master的相关文件来解决问题。但是那样会相对麻烦，没有这种方法简便、有效！
C:\Windows\system32>net stop mssqlserver
The SQL Server (MSSQLSERVER) service is stopping.
The SQL Server (MSSQLSERVER) service was stopped successfully.
C:\Windows\system32>net start mssqlserver
The SQL Server (MSSQLSERVER) service is starting.
The SQL Server (MSSQLSERVER) service was started successfully.
