I've put the code, stripped to a minimal example here:

• https://ldc.godbolt.org/z/fzsx3Tnnn

You can see that the single write + read version of the code works just fine:

pageData[0..4] = [1, 2, 3, 4]
readData[0..4] = [1, 2, 3, 4]

Where here, pageData is the data to be written to a file, and readData is the result of trying to read the freshly written file data.

But if the same code is placed inside of a for loop, suddenly no writes occur:

pageData[0..4] = [0, 0, 0, 0]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 1]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 2]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 3]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 4]
readData[0..4] = [0, 0, 0, 0]

// ...

Does anyone know what is happening here? It's really puzzling.

On Sunday, 28 August 2022 at 22:46:17 UTC, Gavin Ray wrote:

pageData[0..4] = [1, 2, 3, 4]
readData[0..4] = [1, 2, 3, 4]

pageData[0..4] = [0, 0, 0, 0]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 1]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 2]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 3]
readData[0..4] = [0, 0, 0, 0]

pageData[0..4] = [0, 0, 0, 4]
readData[0..4] = [0, 0, 0, 0]

// ...

You probably need to flush the output.

On Monday, 29 August 2022 at 07:04:49 UTC, bauss wrote:

That's a good idea. I gave it a shot, and the following doesn't seem to change anything unfortunately:

void writePage(PageId pageId, ubyte[PAGE_SIZE] pageData)
{
    synchronized (dbIOMutex.writer)
    {
        dbFile.seek(pageId * PAGE_SIZE);
        dbFile.rawWrite(pageData);
        dbFile.flush();
        dbFile.sync();
    }
}

On Sunday, 28 August 2022 at 22:46:17 UTC, Gavin Ray wrote:

Relevant pieces of the code:

class DiskManager
{
    void writePage(PageId pageId, ubyte[PAGE_SIZE] pageData)
    {
        synchronized (dbIOMutex.writer)
        {
            dbFile.seek(pageId * PAGE_SIZE);
            dbFile.rawWrite(pageData);
        }
    }
}

void singleReadWrite()
{
    PageId pageId = 0;

    diskManager.writePage(pageId, pageData);
}

void multiReadWrite()
{
    PageId pageId = 0;

    foreach (i; 0 .. 10)
    {
        diskManager.writePage(pageId, pageData);
    }
}

You never change pageId. So as far as I can tell, you're always seek-ing to the same position, and you just overwrite the same piece of the file again and again.

On Monday, 29 August 2022 at 16:21:53 UTC, ag0aep6g wrote:

Whoops. I guess I missed the point of the question there.

On Monday, 29 August 2022 at 15:52:31 UTC, rikki cattermole wrote:

D'oh!

I didn't even think about the mode:

It must have been the "writing at end of file" bit?

Thanks Rikki. /embarrassed