sync writes any data buffered in memory out to disk. This can include
(but is not limited to) modified superblocks, modified inodes, and delayed
reads and writes. This must be implemented by the kernel; The sync
program does nothing but exercise the sync(2) system call.
The kernel keeps data in memory to avoid doing (relatively slow) disk reads and
writes. This improves performance, but if the computer crashes, data may be
lost or the filesystem corrupted as a result. sync ensures that
everything in memory is written to disk.
sync should be called before the processor is halted in an unusual manner
(e.g., before causing a kernel panic when debugging new kernel code). In
general, the processor should be halted using the shutdown(8) or
reboot(8) or halt(8) commands, which will attempt to put the
system in a quiescent state before calling sync(2). (Various
implementations of these commands exist; consult your documentation; on some
systems one should not call reboot(8) and halt(8) directly.)
On Linux, sync is only guaranteed to schedule the dirty blocks for
writing; it can actually take a short time before all the blocks are finally
written. The reboot(8) and halt(8) commands take this into
account by sleeping for a few seconds after calling sync(2).
This page describes sync as found in the fileutils-4.0 package; other
versions may differ slightly.