int fsync(int fd)
{
fdatasync(fd);
fsyncmetadata();
return ...;
}

int fsync(int fd)
{
fsyncmetadata();
fdatasync(fd);
return ...;
}

• fdatasync() should proceed as usual for files with more than zero links.
• fdatasync() should do nothing for files with zero links.
• fsync() should be implemented as usual through calling fdatasync()

• implementation of sync();
• implementation of loop devices attached to disk files;
• etc.

OK, Stephen, you has pointed where following my idea would really
significantly change the semantics, and it should not do.

So fsync() (but not fdatasync()) should indeed have effect on an inode with
zero links but _only the first time_. Precisely:

1. With every fd should be associated a boolean flag "no_links_committed"
(to save a bit of memory it could be instead implemented e.g. as having -1
(minus one) as the count of links in the fd data structure instead of 0).

2. When a file is unlinked, then if the number of links becomes zero
no_links_commited should be in reset state (or write zero as the count of
links in the fd data structure).

3. When fsync() (but not fdatasync() which is simpler) is called on a file:
- If the number of links is above 0 proceed as usual.
- If the number of links is zero:
* If no_links_commited is false do directory synchronization
(as mentioned by Stephen) but no other synchronization and
then set no_links_committed to true (or number of links to -1 for
a little more efficient impl.)
* If no_links_committed is true, do nothing.

• fdatasync() should have no effect on files with zero links (nor on files with -1 links, see below).
• fd (file descriptor) or inode data structure which contains the number of links of a file should allow additional value -1 (minus one) as the number of links.
• When a file with one link is unlinked, proceed as usual (decrease the number of links to zero).
• When fsync() is called on a file with this count of links equal to there, then:
  1. accomplish synchronization to the disk of the directories which was containing this file (from which this file was removed), but not of the file itself;
  2. decrement the count of links of this fd so making it equal -1.
• When fsync() is called on a fd with -1 links, do nothing. (There are no need to synchronize this inode to disk as it is not a real use visible file, and directories from which it was removed are already synchronized.)