Date: Mon, 19 Oct 1998 11:10:55 -0700 From: John Polstra <jdp@polstra.com> To: obrien@NUXI.com Cc: hackers@FreeBSD.ORG Subject: Re: softupdates and sync Message-ID: <199810191810.LAA14424@austin.polstra.com> In-Reply-To: Your message of "Mon, 19 Oct 1998 11:00:57 PDT." <19981019110057.A28374@nuxi.com>
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> Correctly me if I'm wrong: if you want to know the point at which
> the disks are synced, you need to run it twice, right?
>
> Reason being the 1st ``sync'' simply kicks off the syncing procedure
> and then returns. The 2nd ``sync'' will not return until the
> syncing procedure from the 1st ``sync'' has completed. Thus once
> the 2nd ``sync'' returns, you know your disk buffers are completely
> flushed to disk.
I haven't studied the code, but I know what the documentation
says. It says, "sync() may return before the buffers are completely
flushed." It doesn't say that if you run it a second time, it will
make sure that the first sync() has completed. There's no stated
guarantee of that kind.
I think that whatever historical benefit came from running it three
times was strictly derived from the fact that it takes a few seconds
to type "sync<CR>" three times. It would work just as well to type:
sync
echo hi
echo hi
:-)
When unmounting a filesystem, you don't have to run sync at all.
Unmounting will automatically write out all the dirty buffers.
John
--
John Polstra jdp@polstra.com
John D. Polstra & Co., Inc. Seattle, Washington USA
"Self-knowledge is always bad news." -- John Barth
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