Is execution of sync(8) still required before shutting down linux?
- by Amos Shapira
I still see people recommend use of "sync; sync; sync; sleep 30; halt" incantations when talking about shutting down or rebooting Linux.
I've been running Linux since its inception and although this was the recommended procedure in the BSD 4.2/4.3 and SunOS 4 days, I can't recall that I had to do that for at least the last ten years, during which I probably went through shutdown/reboot of Linux maybe thousands of times.
I suspect that this is an anachronism since the days that the kernel couldn't unmount and sync the root filesystem and other critical filesystems required even during single-user mode (e.g. /tmp), and therefore it was necessary to tell it explicitly to flush as much data as it can to disk.
These days, without finding the relevant code in the kernel source yet (digging through http://lxr.linux.no and google), I suspect that the kernel is smart enough to cleanly unmount even the root filesystem and the filesystem is smart enough to effectively do a sync(2) before unmounting itself during a normal "shutdown"/"reboot"/"poweorff".
The "sync; sync; sync" is only necessary in extreme cases where the filesystem won't unmount cleanly (e.g. physical disk failure) or the system is in a state that only forcing a direct reboot(8) will get it out of its freeze (e.g. the load is too high to let it schedule the shutdown command).
I also never do the "sync" procedure before unmounting removable devices, and never hit a problem.
Another example - Xen allows the DomU to be sent a "shutdown" command from the Dom0, this is considered a "clean shutdown" without anyone having to login and type the magical "sync; sync; sync" first.
Am I right or was I lucky for a few thousands of system shutdowns?
