sync - commit buffer cache to disk.
sync first commits inodes to buffers, and then
buffers to disk.
sync always returns 0.
SVr4, SVID, X/OPEN, BSD 4.3
According to the standard specification (e.g., SVID),
sync() schedules the writes, but may return before
the actual writing is done. However, since version 1.3.20
Linux does actually wait. (This still does not guarantee
data integrity: modern disks have large
bdflush(2), fsync(2), fdatasync(2),