Re: Reality, humor, and history (was Re: MORE CRITICAL FLAWS IN MS WINDOWS EXPLORER

[Stormwalker <bruen () coldrain net>]

Hi Valdis,

  Must be my age...

  The synch call writes memory/cache resident data to the appropriate disk
files, but does not wait to see if all the dirty buffers in memory have
been written to disk before it completes. There is no good way to know if
all have files have been updated, except to synch a few times. The haltsys
command (Unix V) and shutdown -h issue several synch calls to take care of
the problem. Unix today generally issues an update command which calls
synch every 30 seconds or so in case of a crash or other sudden,
unexpected shutdown (eg fsflush in SVR4). And then there is the potential 
problem of remote disks, which can cause time delays.
 
The metadata inconsistencies are handled differently, partly by the write 
order and partly by fsck.

              cheers, bob

(*) My all-time favorite "Close, but no ceee-gar" was the advice column
for a Unix journal where the author *remembered* the old "3 syncs before
halt" adage - but got it Very Wrong by advising "sync;sync;sync;halt".  
Bonus points if you can remember (a) the *original* reason for the advice
*and* (b) how this version was Very Wrong (there's *multiple* answers for
this one ;)


-- 
Dr. Robert Bruen
Cold Rain Technologies 
http://coldrain.net
+1.802.579.6288

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