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From: jrw@hropus.UUCP (Jim Webb)
Newsgroups: comp.unix.questions,comp.unix.wizards,comp.databases
Subject: Re: O_SYNC option in SVR3 open system call
Message-ID: <777@hropus.UUCP>
Date: Tue, 11-Nov-86 17:18:18 EST
Article-I.D.: hropus.777
Posted: Tue Nov 11 17:18:18 1986
Date-Received: Wed, 12-Nov-86 04:52:39 EST
References: <5922@gatech.EDU> <9119@sun.uucp>
Organization: Bell Labs, Holmdel, NJ
Lines: 27
Xref: mnetor comp.unix.questions:18 comp.unix.wizards:30 comp.databases:4

> > 	O_SYNC	When opening a regular file, this flag affects subsequent
> > 		writes. If set, each write (2) will wait for both the file
> > 		data and file status to be physically updated.
> > 
> > Does anybody out there knows if this wait is for real or is the same BSD
> > idea of the "fflush" in which blocks are merely "scheduled" and the call 
> > returns as if they were actually written to the device. 
> 
> The wait is for real.

And this has been an (albeit undocumented) flag in all vax and 3bx versions of
AT&T System V.

Not to go into too much detail (my hands would get probably get crushed
instead of just slapped ;-) but the kernel has two write routines, one that
just schedules the disk block to be written and another that waits for the
actually writing before returning.  If O_SYNC is set, then the former is
called.

To find out whether your version of UNIX supports this, grep for SYNC in
/usr/include/sys/file.h.  That will give you the octal value of the flag,
if it is available.   Then, I guess to see if the kernel recognizes it,
time writes with and without the O_SYNC flag set.  The latter writes should
be a good bit faster.
-- 
Jim Webb             "Out of phase--get help"          ...!ihnp4!hropus!jrw
		"Use the Force, Read the Source"