Sync

The sync system call writes dirty buffers to the file system or disk. The call will block until all dirty buffers are written.

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.