Re: soft update vs journaling?

[John Richard Moser <nigelenki@comcast.net>]

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Michael Loftis wrote:
> 
> 
> --On January 22, 2006 1:42:38 AM -0500 John Richard Moser
> <nigelenki@comcast.net> wrote:
> 
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>> So I've been researching, because I thought this "Soft Update" thing
>> that BSD uses was some weird freak-ass way to totally corrupt a file
>> system if the power drops.  Seems I was wrong; it's actually just the
>> opposite, an alternate solution to journaling.  So let's compare notes.
> 
> 
> I hate to say it...but in my experience, this has been exactly the case
> with soft updates and FreeBSD 4 up to 4.11 pre releases.
> 
> Whenever something untoward would happen, the filesystem almost always
> lost files and/or data, usually just files though.  In practice it's
> never really worked too well for me.  It also still requires a full fsck
> on boot, which means long boot times for recovery on large filesystems.

You lost files in use, or random files?

Soft Update was designed to assure file system consistency.  In typical
usage, when you drop power on something like FAT, you create a 'hole' in
the filesystem.  This hole could be something like files pointing to
allocated blocks belonging to other files; or crossed dentries; etc.  As
you use the file system, it simply accepts the information it gets,
because it doesn't look bad until you look at EVERYTHING.  The effect is
akin to repeatedly sodomizing the file system in this newly created
hole; you just cause more and more damage until the system gives out.
The system makes allocations and decisions based on faulty data and
really, really screws things up.

The idea of Soft Update was to make sure that while you may lose
something, when you come back up the FS is in a safely usable state.
The fsck only colors in a view of the FS and frees up blocks that don't
seem to be allocated by any particular file, an annoying but mostly
harmless side effect of losing power in this scheme.
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