const int FILE_FLAG_NO_BUFFERING = 0x20000000;
using (var stream = new FileStream(name, FileMode.Open, FileAccess.Read, FileShare.Read, buffer.Length,
    (FileOptions) FILE_FLAG_NO_BUFFERING | FileOptions.WriteThrough))
{
   ....
}

VC>Надо как то самому получить SafeFileHandle и передать его в констутор

VC>public FileStream(SafeFileHandle handle, FileAccess access, int bufferSize, bool isAsync);
VC>

FileStream OpenFileStreamWithoutCacheOnLinux(int bufferSize)
{
    var openFlags = /* Mono.Unix.Native.OpenFlags.O_DIRECT | */ Mono.Unix.Native.OpenFlags.O_RDONLY;
    int handle = Mono.Unix.Native.Syscall.open(TempFile, openFlags);
    IntPtr rawHandle = new IntPtr(handle);
    SafeFileHandle fh = new SafeFileHandle(rawHandle, false);
    FileStream fs = new FileStream(fh, FileAccess.Read, bufferSize, false);
    return fs;
}

Unhandled Exception: System.IO.IOException: Invalid argument
   at System.IO.FileStream.CheckFileCall(Int64 result, Boolean ignoreNotSupported)
   at System.IO.FileStream.ReadNative(Span`1 buffer)
   at System.IO.FileStream.ReadSpan(Span`1 destination)
   at System.IO.FileStream.Read(Byte[] array, Int32 offset, Int32 count)

int bufferFile
string fileName;

var openFlags = /* Mono.Unix.Native.OpenFlags.O_DIRECT | */ Mono.Unix.Native.OpenFlags.O_RDONLY;
int handle = Mono.Unix.Native.Syscall.open(fileName, openFlags);
IntPtr rawHandle = new IntPtr(handle);
SafeFileHandle fh = new SafeFileHandle(rawHandle, false);
FileStream fs = new FileStream(fh, FileAccess.Read, bufferSize, false);

Disk: /ssd, Size: 100M, Block: 4096 bytes
O_DIRECT is present
[Completed]  100.0% 00:00:08     12M/s Allocate
[Completed]  100.0% 00:00:02   37.8M/s Sequential read
[Completed]  100.0% 00:00:06   15.8M/s Sequential write
[Completed]  100.0% 00:00:20 6196.6K/s Random Read, 1 thread
[Completed]  100.0% 00:00:20 1252.4K/s Random Write, 1 thread
[Completed]  100.0% 00:00:20 6521.9K/s Random Read, 16 threads
[Completed]  100.2% 00:00:20 1306.7K/s Random Write, 16 threads

Disk: /ssd, Size: 100M, Block: 4096 bytes
O_DIRECT is disabled
[Completed]  100.0% 00:00:08   11.9M/s Allocate
[Completed]  100.0% 00:00:02   37.9M/s Sequential read
[Completed]  100.0% 00:00:04   16.6M/s Sequential write
[Completed]  100.0% 00:00:20  181.8M/s Random Read, 1 thread
[Completed]  100.0% 00:00:20 1448.9K/s Random Write, 1 thread
[Completed]  100.0% 00:00:20  769.3M/s Random Read, 16 threads
[Completed]  100.1% 00:00:20 1489.6K/s Random Write, 16 threads