Messages in this thread | | | From | Miquel van Smoorenburg <> | Subject | Re: system.map | Date | Thu, 3 Jan 2002 09:43:41 +0000 (UTC) |
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In article <200201022039.g02KdPSr022018@svr3.applink.net>, Timothy Covell <timothy.covell@ashavan.org> wrote: >On Wednesday 02 January 2002 14:19, Kilobug wrote: >> > 5. sync;sync;shutdown -r now >> >> Is there any particular reason for this double sync ? One isn't enough ? >> (And is sync even needed with shutdown, all should be synced when >> filesystems are unmounted or remounted read-only, am I wrong ? ) > >The double sync is tradition.
The double sync is because traditionally, sync was asyncronous- it told the kernel 'flush write cache to disk' but it returned immidiately. That is why people were told to sync 3 times - by the time you had typed 'sync' for the third time on your 300 baud lineprinter console the system was done flushing.
sync;sync;sync;reboot is NOT what was used, it was:
# sync # sync # sync # reboot
Anyway, Linux sync is different. It *is* synchronous. However syncing and then doing a hard reboot is not recommended, you really need to unmount all filesystems first, and remount root read-only. The standard shutdown sequence does all of this for you and typing sync before shutdown -r now is completely useless.
>SysV init scripts should sync things, >but "sync;sync;reboot" or "sync;sync;halt" are not so nice in how >they go down; so it's a case of being extra careful. I don't use >linux all the time, and some of the other unices are less tolerant. >(For example, on a sun box, I would prefer a double sync before I >"<stop>-a".)
If you're going to halt or reboot a system *hard* by using the reset button or <stop>-a without calling shutdown you better sync before you do that yes, and then you're still not guaranteed free of fs corruption - at least kill all processes first to prevent some process writing to disk in between the sync and the halt/reset.
Mike.
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