Assume that you use Microsoft SQL Server 2014 that has databases with memory-optimized objects. When you restart SQL Server, it restarts successfully, and you may occasionally receive the following messages in the error log:
[ERROR] Unable to delete file <File Path>\<File Name>.dll for reason 5. File renamed to <File Path>\<File Name>.old_dll. This issue may be due to a previous failure to unload memory optimized table DLLs.
The issue occurs because of a timing issue. On server restart for databases with memory-optimized objects, the server is sometimes unable to delete old DLLs for memory-optimized tables. The server can continue the restart by renaming the old DLLs. But it logs a message to the error log for which there is no user action required or possible.
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Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.