From owner-freebsd-hackers  Tue Feb  6 11:56:33 2001
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Date: Tue, 6 Feb 2001 11:56:12 -0800 (PST)
From: Matt Dillon <dillon@earth.backplane.com>
Message-Id: <200102061956.f16JuCb65538@earth.backplane.com>
To: Andre Oppermann <oppermann@monzoon.net>
Cc: Mike Silbersack <silby@silby.com>,
	Rik van Riel <riel@conectiva.com.br>,
	Poul-Henning Kamp <phk@critter.freebsd.dk>,
	Charles Randall <crandall@matchlogic.com>,
	Dan Phoenix <dphoenix@bravenet.com>,
	Alfred Perlstein <bright@wintelcom.net>,
	Jos Backus <josb@cncdsl.com>, freebsd-hackers@FreeBSD.ORG
Subject: Re: soft updates and qmail (RE: qmail IO problems)
References: <Pine.BSF.4.31.0102061149180.14899-100000@achilles.silby.com> <200102061759.f16Hxv662437@earth.backplane.com> <3A805137.230E0A0D@monzoon.net>
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:
:This information is in fact correct. Have a look at the FreeBSD link(2)
:man page:
:
:LINK(2)                   FreeBSD System Calls Manual                 
:LINK(2)

    Andre, I think there *might* be a dozen people in the world that
    understand UFS/FFS better then I do, but none of them have posted to
    this thread.

    Believe me when I say that there is no metadata ordering guarentee
    in the case of a crash.  Yes a standard UFS/FFS will do synchronous
    metadata updates for certain operations.  No, this does not guarentee
    metadata ordering.

    There has been talk of providing system calls to allow user programs
    to request ordering semantics for certain operations, but nobody has 
    actually implemented anything.  Most of the discussion has been centered
    on having calls to guarentee file write ordering between a set of
    open descriptors for databases.

    As I said, a journaled filesystem can theoretically make metadata
    ordering guarentees, but actually doing so creates massive performance
    and scaleability issues that aren't apparent until you really start
    pounding the filesystem.  It *CAN* be done efficiently, but only if 
    you have a significant amount of non-volatile memory store to hold
    the journal.   ReiserFS might do it right now, as a side effect, but if
    it does it faces serious scaleability issues.

    Softupdates can also theoretically order [meta]data, using dependancies.
    It is a very difficult problem to solve and it doesn't do it now.  All
    softupdates does is guarentee filesystem consistency in the case of a
    crash and certain guarentees for what will be crash-recoverable when you
    do an fsync().

						-Matt



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