softdep && logging

[Matthew Mondor <mm_lists%pulsar-zone.net@localhost>]

Hello,

Considering these paper/slides presented at BSDCon 2010:
http://www.mckusick.com/publications/suj.pdf
http://www.mckusick.com/publications/suj-slides.pdf

It appears that there now is an option to use a combination of soft
updates and journalling on FreeBSD.  Our softdep code was removed in
-current, considering the burden of fixing it when an alternative,
WAPBL became available.

However, this probably means that FreeBSD would also have a less buggy
and MP-safe softdep implementation which might possibly be portable to
NetBSD (although possibly invasive code, as our softdep was), and that
this alternative used with logging might potentially result in less I/O
overhead than WAPBL as well as better metadata referencial integrity.

As I recall, with WAPBL (as with FFS without softdep) it's possible for
extra blocks (potentially garbage) to remain attached to files in the
event of a crash, which is a problem of FFS itself, but which softdep
helped to mitigate.

After I read about this today, I thought I'd share the links, in case
interested parties want to consider trying future alternatives to
FFS+WAPBL or LFS on NetBSD.

Thanks,
-- 
Matt