Re: softdep : why not the default

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Wed, Nov 23, 2011 at 02:43:01PM -0500, Dieter BSD wrote:
> > Up until then softdeps was needed for the speed but ffs with or
> > without softdeps usually meant a long delay to fsck after any
> > unclean shutdown.
> 
> FFS w/ softdeps allows background fsck (after going multiuser).
> Works fine on FreeBSD. ?NetBSD needs to add background fsck.

Since NetBSD is a volunteer project, if NetBSD "needs" to do something, you
are certainly entirely free to step forward and do it.

Except in highly exceptional conditions -- where one would not want to run
fsck in the background anyway -- journalled FFS filesystems do not require
fsck at all.  The "fsck" utility runs, but it simply replays the journal
and immediately exits.

> > ``[---] I believe that's simply because the softdep code is difficult
> > and time consuming to maintain. Some of the bugs that I know of are
> > easy to reproduce and particularly difficult to fix.''
> 
> Why is this a problem for NetBSD but not for FreeBSD?

The FreeBSD project chose to spend a huge amount of effort fixing and
working around the limitations of the original softdep code.  Volunteer
project, remember?  Further, they were willing to live for years with
inconvenient restrictions such as having to recommend that soft dependencies
never be enabled on the root filesystem (this workaround may still be present
in their install utilities, in fact).  We chose a different path.

As Charles Hannum pointed out years ago, it would be great to have a sane,
filesystem-independent framework that could draw a DAG of dependencies and
schedule updates, but the FFS softdep code isn't it.

-- 
Thor Lancelot Simon                                    tls%panix.com@localhost
  "All of my opinions are consistent, but I cannot present them all
   at once."    -Jean-Jacques Rousseau, On The Social Contract