Disable cacheing before reading from a file using Swift

Hello,

I am trying to implement some sort of minimal I/O benchmarking feature. I can calculate the write speeds with the following code:

guard var handle = FileHandle(forUpdatingAtPath: url.path) else { return }

var writtenData: UInt64 = 0
var totalWrittenData: UInt64 = 0
var writeSpeed: UInt64 = 0

let data = Data(repeating: 0, count: 16 * 1024 * 1024) // 16 MB test block
let startTime = Date.now.timeIntervalSince1970

while Date.now.timeIntervalSince1970 - startTime < 5.0
{
    handle.write(data)
    try? handle.synchronize()self.totalWrittenData += blockSize

    let duration = Date.timestamp - self.startTime
    writeSpeed = UInt64(Double(self.totalWrittenData) / duration)

    if writtenData > blockSize * 16
    {
         writtenDara = 0
         try? handle.seek(toOffset: 0)
    }
}

// On my PCE-Express SSD this results in roughly 1800MB/s. This value matches the value reported by other benchmarking apps.

// Remove everything at the end of the file
try? handle.truncate(atOffset: blockSize)

try? handle.synchronize()
try? handle.seek(toOffset: 0)

var index = 0
while index < 5
{
     autoreleasepool
    {
        var startTime = Date.timestamp
        let bytes = try? handle.readToEnd()
        let duration = Date.timestamp - startTime

         Swift.print(bytes?.count, duration)
         try? handle.seek(toOffset: 0)
     }

     index += 1
}

The code works - in theory. Although I really like that the operating system and drivers do some cacheing to optimise performance - in my use case I don't want files to be cached. The read results are always around 8GB/s for my PCE-Express SSD and also for some older USB Sticks, so the data seems to come directly from the memory.

I found a couple of other threads which led me to some older C functions, specifically this code:

// Open file.
let fd = fopen(fileUrl.path, "r")

// Find the end
fseek(fd, 0, SEEK_END)

// Count bytes
let fileByteSize = ftell(fd)

// Return to start
fseek(fd2, 0, SEEK_SET)

// Disable cache
setvbuf(fd2, nil, _IONBF, 0)

// Find the end
fseek(fd2, 0, SEEK_END)

let readBytes = fread(pointer, 1, fileByteSize, fd2)

readBytes += UInt64(readBytes.count)

When executing this code (or parts of it), it also works fine - but the data is also coming from the cache.

What am I missing? I also tried the FileManager. Of course I am not an expert, however I think that the FileManager is "just" a wrapper around all those C functions.

Is there any way to ignore the cacheing mechanism and tell macOS / the driver to read the data directly from the drive?

Regards, Sascha