When does O_SYNC have an effect?

Question

I was looking into an issue I had with Mender, where the installation progress (which is about copying a file on a block device) is not reported correctly.

My feeling is that it's about the kernel page cache: the progress bar says "100%" when the code has read the whole image, but that does not mean that the kernel is done writing it.

More specifically, Mender calls n, err := io.Copy(dev, image), which returns after the kernel is done writing. But the progress bar is linked to the "image" Reader, which is fully read tens of seconds before io.Copy returns.

Because the file is opened with flags here, I naively thought that I just had to set flag |= O_SYNC, so that io.Copy(dev, image) would not read image faster than it writes to dev.

But setting O_SYNC does not make a difference.

It is not clear to me if O_SYNC is merely a hint (so I cannot count on it), if it could be that I am missing something on my device (say, I forgot a kernel option on my Raspberry Pi and therefore O_SYNC is useless), or if I just misunderstood what O_SYNC does?

EDIT: I also tried to set O_SYNC | O_DIRECT (though O_DIRECT is apparently not exposed in Go and so I did O_SYNC | 0o40000), but I got the following error when opening the block device:

Opening device: /dev/mmcblk0p2 for writing with flag: 1069058
Failed to open the device: "/dev/mmcblk0p2": open /dev/mmcblk0p2: not a directory

Answer 1

Summarizing the comments:

1. The main issue is that the progress bar is decorating the reader (as Yotam Salmon noted), not the writer; the delay is on the side of the writer.

2. On most Linux systems, O_DIRECT is indeed 0o40000, but on ARM (including Raspberry Pi) it is 0o200000, with 0o40000 being O_DIRECTORY. This explains the "not a directory" error.

3. O_SYNC is in fact the bit you want, or you can simply issue an fsync system call (use Flush if appropriate, and then Sync, as noted in When to flush a file in Go?). The O_SYNC bit implies an fsync system call as part of each write system call.

Fully synchronous I/O is a bit of a minefield: some devices lie about whether they've written data to nonvolatile storage. However, O_SYNC or fsync is the most guarantee you'll get here. O_DIRECT is likely irrelevant since you're going directly to a device partition /dev file. O_SYNC or fsync may be passed through to the device driver, which may do something with it, which may get the device to write to nonvolatile storage. There's more about this in What does O_DIRECT really mean?