Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!lotus!esegue!johnl
From: johnl@esegue.segue.boston.ma.us (John R. Levine)
Newsgroups: comp.unix.questions
Subject: Re: sparse files
Message-ID: <1989Dec13.172444.27531@esegue.segue.boston.ma.us>
Date: 13 Dec 89 17:24:44 GMT
References: <21581@adm.BRL.MIL> <235@dg.dg.com> <2700@auspex.auspex.com>
Reply-To: johnl@esegue.segue.boston.ma.us (John R. Levine)
Organization: Segue Software, Cambridge MA
Lines: 18

In article <2700@auspex.auspex.com> guy@auspex.auspex.com (Guy Harris) writes:
>At least the first version of AIX for the RT PC claimed in its documentation
>that it had an "fclear()" call to punch holes in files; ...

Yes, it did, I put it there myself.  The semantics were basically the same
as a write of a buffer of zeros but the implementation promised to make holes
where it could.  In retrospect, it was a mistake.  One of my coworkers
proposed that we make write() a little smarter and have it notice when the
buffer was all-zero and write a hole.  At the time I thought that would be
too slow, but he correctly pointed out that few buffers that are not entirely
zero start with a lot of zeros, so in most cases the overhead would be tiny.
Fixing write() would have the enormous advantage of automatically making
cp, cpio, tar, dump/restor, and everything else do the right thing with
sparse files.
-- 
John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 864 9650
johnl@esegue.segue.boston.ma.us, {ima|lotus|spdcc}!esegue!johnl
"Now, we are all jelly doughnuts."