How to ignore cache when repeatedly reading from disk - swift

Question

I am writing an app that contains a small benchmark for I/O operations.
For write operations, I am using a 'FileHandle' which works pretty well. I am testing my old USB stick and my calculation results in values of roughly 20MB/s which seems correct.
However, when reading, the values jump up to 8 GB/s. Although I would love to have an USB stick that fast...I think this has to do with some sort of cacheing.
Here is the code that I am using (some bits were removed):
guard let handle = FileHandle(forUpdatingAtPath: url.path) else { return }
let data = Data(repeating: 0, count: 2 * 1024 * 1024)
var startTime = Date.timestamp
// Write Test
while Date.timestamp - startTime < 5.0
{
handle.write(data)
try? handle.synchronize()
// ...
}
// Go back to beginning of file.
try? handle.seek(toOffset: 0)
// Remove everything at the end of the file
try? handle.truncate(atOffset: blockSize)
startTime = Date.timestamp
// Read Test
while Date.timestamp - startTime < 5.0
{
autoreleasepool
{
if let handle = try? FileHandle(forReadingFrom: fileUrl), let data = try? handle.readToEnd()
{
let count = UInt64(data.count)
self.readData += count
self.totalReadData += count
handle.close()
}
// I also tried FileManager.default.contents(atPath: ) - same result
}
}
I also tried this piece of code (it's either from Martin R. here on SO or from Quinn on the Apple forums):
let fd = open(fileUrl.path, O_RDONLY)
_ = fcntl(fd, F_NOCACHE, 1)
var buffer = Data(count: 1024 * 1024)
buffer.withUnsafeMutableBytes { ptr in
let amount = read(fd, ptr.baseAddress, ptr.count)
self.readData += UInt64(amount)
self.totalReadData += UInt64(amount)
}
close(fd)
The code itself works...but there is still cacheing.
TL;DR How can I disable cacheing when writing to and reading from a file using Swift?
Regards