Difference between “none” and “swap” as a mountpoint for swap in fstab

async is the opposite of sync, which is rarely used. async is the default, you don't need to specify that explicitly.

The option sync means that all changes to the according filesystem are immediately flushed to disk; the respective write operations are being waited for. For mechanical drives that means a huge slow down since the system has to move the disk heads to the right position; with sync the userland process has to wait for the operation to complete. In contrast, with async the system buffers the write operation and optimizes the actual writes; meanwhile, instead of being blocked the process in userland continues to run. (If something goes wrong, then close() returns -1 with errno = EIO.)

SSD: I don't know how fast the SSD memory is compared to RAM memory, but certainly it is not faster, so sync is likely to give a performance penalty, although not as bad as with mechanical disk drives. As of the lifetime, the wisdom is still valid, since writing to a SSD a lot "wears" it off. The worst scenario would be a process that makes a lot of changes to the same place; with sync each of them hits the SSD, while with async (the default) the SSD won't see most of them due to the kernel buffering.

In the end of the day, don't bother with sync, it's most likely that you're fine with async.

A bind mount is equivalent to the original. There isn't one that's marked as the original and one that's marked as a copy. Bind mounts are like hard links in this respect, not like symbolic links.

Since GNU coreutils 8.21 (if I read the changelog correctly), df strives to report each filesystem only once. Older versions included one entry per non-ignored mount point, so a bind mount would result in multiple entries. df shows the latest mount point for each filesystem, as far as it can determine. In your case, df reports /opt, not /data, because /opt was added last.

Pass the option -a to get even recent coreutils to report all entries for each filesystem. This also causes filesystems that would normally be ignored to be listed, if you don't pass a path argument. If you do pass a path argument, I think df never reports anything but the latest mount point.

If you don't want to alias df to something else, you'll either have to replace df by a custom implementation or have to live with df reporting something different from what you'd like. If you need the earliest mount point rather than the latest one, a sane solution would be to call df -a and retain only the first match for the device that you're interested in.