io: Add DISABLE_POSIX_KERNEL_AIO knob to use EIO instead of Kernel AIO

- Some Linux filesystems don't support O_DIRECT which is required by
Kernel AIO to function properly. Instead of using O_SYNC, EIO is
much better options in terms of performance penalty.
- Some systems may not support AIO at all. Eg. Windows Subsystem for
Linux.

FIXES #842
RELATED #274

fdbrpc/Net2FileSystem.cpp

@@ -58,7 +58,12 @@ Future< Reference<class IAsyncFile> > Net2FileSystem::open( std::string filename
 
 	Future<Reference<IAsyncFile>> f;
 #ifdef __linux__
-	if ( (flags & IAsyncFile::OPEN_UNBUFFERED) && !(flags & IAsyncFile::OPEN_NO_AIO) )
+	// In the vast majority of cases, we wish to use Kernel AIO. However, some systems
+	// dont properly support don’t properly support kernel async I/O without O_DIRECT
+	// or AIO at all. In such cases, DISABLE_POSIX_KERNEL_AIO knob can be enabled to fallback to
+	// EIO instead of Kernel AIO.
+	if ((flags & IAsyncFile::OPEN_UNBUFFERED) && !(flags & IAsyncFile::OPEN_NO_AIO) &&
+	    !FLOW_KNOBS->DISABLE_POSIX_KERNEL_AIO)
 		f = AsyncFileKAIO::open(filename, flags, mode, NULL);
 	else
 #endif

flow/Knobs.cpp

@@ -79,6 +79,7 @@ FlowKnobs::FlowKnobs(bool randomize, bool isSimulated) {
 	init( MIN_SUBMIT,                                           10 );
 
 	init( PAGE_WRITE_CHECKSUM_HISTORY,                           0 ); if( randomize && BUGGIFY ) PAGE_WRITE_CHECKSUM_HISTORY = 10000000;
+	init( DISABLE_POSIX_KERNEL_AIO,                              0 );
 
 	//AsyncFileNonDurable
 	init( MAX_PRIOR_MODIFICATION_DELAY,                        1.0 ); if( randomize && BUGGIFY ) MAX_PRIOR_MODIFICATION_DELAY = 10.0;

flow/Knobs.h

@@ -98,6 +98,7 @@ public:
 	int MIN_SUBMIT;
 
 	int PAGE_WRITE_CHECKSUM_HISTORY;
+	int DISABLE_POSIX_KERNEL_AIO;
 
 	//AsyncFileNonDurable
 	double MAX_PRIOR_MODIFICATION_DELAY;