Re: msync() behaviour broken for MS_ASYNC, revert patch?

[Linus Torvalds <torvalds@osdl.org>]

On Sat, 11 Feb 2006, Nick Piggin wrote:
> > 
> > Your pattern would actually be
> > 
> > 	.. dirty offset 100-200 ..
> > 	fadvice(fd, 100, 200, FADV_WRITE_START);
> > 
> > 	.. dirty offset 200-300 ..
> > 	fadvice(fd, 200, 300, FADV_WRITE_START);
> > 
> > 	.. dirty offset 300-400 ..
> > 	fadvice(fd, 300, 400, FADV_WRITE_START);
> > 
> > 	.. dirty offset 400-415 .. (for the next transaction)
> > 
> 
> - IOW if the app or OS crashed here it would be possible to see 400-415 on
> the disk and none of the previous transactions (assuming we don't know
> the page size).

If the app/OS crashed here, nothing would matter. We haven't committed 
anything at all yet. We've just started the IO. What is at 400-415 simply 
doesn't matter, because nobody would have any reason to look at it.

(Besides, it's not at all clear that 400-415 would or would not be on 
disk. It depends on entirely on timing and buffering of the IO system at 
that point - the fact that its dirty in memory doesn't mean that it ever 
made it into the IO buffer that was started).

> > 	fadvice(fd, 100, 400, FADV_JUST_WAIT); (for the previous one)

This is the one that waits for it to finish, so _now_ we can update the 
pointers (elsewhere) to that log (and if the app/OS crashes before that, 
nobody will even know about it).

See?

> I'm not convinced. You above example was bogus.

No, your understanding was incomplete. I'm talking about just parts of a 
much bigger transaction. 

A single write on its own is almost never a transaction unless your system 
is _purely_ log-based (which it could be, of course. Not in my example).

		Linus
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