In article <1991Sep04.194822.22...@uvmark.uucp>, t...@uvmark.uucp (Tom Rauschenbach) writes:
|> I've noticed that most Un*x folks type the sync command three times before |> shutting down a system. Is there a folkloric reason for this ? A technical |> one. I have a guess, and I wonder if it can be confirmed or refuted.
Quoting from sync(2) from the Ultrix 4.1 man pages...
The sync system call causes all information in memory that should be on disk to be written out. This includes modified super- blocks, modified i-nodes, and delayed block I/O.
Programs that examine a file system, for example, fsck or df, use the sync system call. The writing, although scheduled, is not necessarily complete upon return from sync.
The second sentence of the second paragraph suggests why three syncs are required before shutting down a Unix system.
Brian.
-- Brian Ellis Computing Services Centre Domain: Brian.El...@vuw.ac.nz Victoria University of Wellington Bang paths... grrrr!!!!! P.O Box 600, New Zealand. What! - no cute .sig ???
t...@uvmark.uucp (Tom Rauschenbach) writes: >I've noticed that most Un*x folks type the sync command three times before >shutting down a system. Is there a folkloric reason for this ? A technical >one.
One for me, one for thee, and one for the pot!
As others have remarked, it's not the case that when you get your prompt back, the disks have been synced. Old-time unix folks tell me that the correct method is to type sync once, then listen or watch for the disk, and wait until it stops. I suppose that the extra two syncs give you something to do while waiting.
I heard a theory that the first sync schedules the writing, and that the second sync won't return until the writing actually scheduled by the first sync is completed, but I'm very sure that this is false. It wouldn't explain why people do three or more, anyway.
ajr
p.s. !@#$%^& a/ux usually gets fsck errors upon the next boot no matter WHAT you do, but it's abnormal.
I've noticed that most Un*x folks type the sync command three times before shutting down a system. Is there a folkloric reason for this ? A technical one. I have a guess, and I wonder if it can be confirmed or refuted.
-- Tom Rauschenbach "I see nobody on the road," said Alice. "I only wish I had such eyes," the King remarked in a fretful tone. "To be able to see Nobody ! And at that distance too !" ..uunet!merk!uvmark!tom
In article <1991Sep04.194822.22...@uvmark.uucp> t...@uvmark.uucp (Tom Rauschenbach) writes: >I've noticed that most Un*x folks type the sync command three times before >shutting down a system. Is there a folkloric reason for this ? A technical >one. I have a guess, and I wonder if it can be confirmed or refuted.
I'm not sure about other UNIX admin's, but I got started doing three sync commands because that was what was suggested in the SCO Xenix manual.
In reality, all you should need is one, but better safe then sorry, we run three. I've seen some do more than three before a shutdown. Want to be sure that everything is closed and written to disk before powering down a Unix system.
-- (Steve Kosloske) | "A year working in Artificial ------------------------------------| Intelligence is enough to make Internet: z...@csd4.csd.uwm.edu | one believe in God." - UUCP: uunet!z...@csd4.csd.uwm.edu | Alan Perlis
In article <1991Sep04.194822.22...@uvmark.uucp> t...@uvmark.uucp (Tom Rauschenbach) writes: >I've noticed that most Un*x folks type the sync command three times before >shutting down a system. Is there a folkloric reason for this ? A technical >one. I have a guess, and I wonder if it can be confirmed or refuted.
The reason i know for doing more than a single sync is that sync is performed asynchonously by the kernel. When the command returns the sync operation is not yet finished. So If you halt immediatly after that you would still have incorrect filesystems on disk. When memories were small the sync would be completed by the time you had typed anything, so why not a second sync?
A second sync would not make your filesystems any better because there will not be found any blocks in the buffer cache that need syncing.
When memories, and buffer caches, grew bigger this was no guarantee anymore. I heard about a Convex that spend tens of seconds syncing after doing lots of installaion work single user. Modern unixes, at least bsd's, always sync when you halt them, even when they panic. But i still have the old sync, sync, jalt on my fingertips.
tom ploegmakers NIKHEF/K-CSG (t...@nikhefk.nikhef.nl)
po.box 4395, 1009 AJ Amsterdam, the Netherlands. phone: -31 20 5922035
In article <1991Sep4.214231.29...@uwm.edu> z...@csd4.csd.uwm.edu
(Steven M Kosloske) writes:
I'm not sure about other UNIX admin's, but I got started doing three sync commands because that was what was suggested in the SCO Xenix manual.
In reality, all you should need is one, but better safe then sorry, we run three. I've seen some do more than three before a shutdown. Want to be sure that everything is closed and written to disk before powering down a Unix system.
From the ultrix manual pages:
Name: sync(1) - update the super block Description: The sync command executes the sync system primitive. The sync command can be called to insure all disk writes have been com- pleted before the processor is halted in a way not suitably done by reboot(8) or halt(8).
See sync(2) for details on the system primitive.
Name: sync(2) - update super-block Description: The sync system call causes all information in memory that should be on disk to be written out. This includes modified super- blocks, modified i-nodes, and delayed block I/O.
[...]
# The writing, although scheduled, is not # necessarily complete upon return from sync.
This last paragraph is the important one. The first sync only asks the system to start flushing the cache, so, you need a certain time for the cache to be clear. The next two syncs are called to give the system time to flush.
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Sex is not the answer. Sex is the question. "Yes" is the answer.
The sync forces a write to the physical disk, and this takes a certain amount of time. Back in the days of ASR33s, it took a fairly significant amount of time to type "sync" three times because one had to litterally pound on the tiny cynlindrical keys. Most people thought that the amount of time necessary to type three sync commands was sufficient for the system to flush its buffers to disk.
Either that, or three is just a nice number. Afterall, there are three stooges, three (talented) Marx brothers --- few people remember Zeppo, and even fewer have even *heard* of Gummo ---, three branches of government in the US, three ... ;-)
In article <1991Sep4.200143.13...@jarvis.csri.toronto.edu>, fl...@dgp.toronto.edu (Alan J Rosenthal) writes:
> p.s. !@#$%^& a/ux usually gets fsck errors upon the next boot no matter WHAT > you do, but it's abnormal.
I used to have this problem with A/UX. I tracked the problem down to a bad SCSI cable. Since replacing the cable, I haven't had any problems even with one blackout, three resets after panics, and several shutdowns. -- Eric Dittman Texas Instruments - Component Test Facility ditt...@skitzo.csc.ti.com ditt...@skbat.csc.ti.com
Disclaimer: I don't speak for Texas Instruments or the Component Test Facility. I don't even speak for myself.
