> The only way to really be sure is to check your files after
    > writing them that they match.

This is assuming that the underlying block device would forcibly flush those queued writes to disk and then re-read them again rather than just serve them up directly from the pending write queue directly without flushing them first.

You generally can't make that assumption about a black box, so reading back your writes guarantees nothing.

Unless you're intimately familiar with your underlying block device you really can't guarantee anything about writes going to physical hardware. All you can do is read its documentation and hope for the best.

If you need a general hack to that's pretty much guaranteed to flush your writes to a physical disk it would be something like:

    After your write, append X random bytes to a file where X is
    greater than your block device's advertised internal memory, then
    call fsync().

Even then you have no guarantees that those writes wouldn't be flushed to the medium while leaving the writes you care about in the block device's internal memory.