The sync system call writes dirty buffers to the file system or disk.
The progress of
sync can be viewed indirectly by:
# grep -e Dirty: -e Writeback: /proc/meminfo
If something is wonky with your disk or network share,
sync may hang. If a reboot is issued, the reboot will also hang when attempting to call
sync. In such circumstances, it might be desirable to do a force reboot (
reboot -f -f) or by rebooting using Sysrq both of which foregoes syncing.