freebsd stable - Sep 2006 - gjournal and Softupdates

I've just watched over some of the gjournal threads.

My main question now is, whats the difference from gjournal and 
softupdates in case of reability ?
Wasn't SU design to make the use of journals needless? As far i 
remember, SU was designed to write in the cache in such a way, that 
whenever the system crashed, the FS is always consistent. Only bgfsck 
has todo a snapshot and cleanup "unused" space that got lost cause the
SU did not finish as the crash occured.

Maybe someone can give me some light into that :). I always tought that 
*BSD don't need a journaling FS as it has already SU

Greetings

Teufel

bsd@kuehlbox.de wrote:
>  Only bgfsck
> has todo a snapshot and cleanup "unused" space that got lost
cause the
> SU did not finish as the crash occured.
> 
> Maybe someone can give me some light into that :). I always tought that 
> *BSD don't need a journaling FS as it has already SU
Soft-updates was a good idea that was implemented a bit too late (with 
regards to development in journaling and file system technologies), and 
now there are several problems with it:

- [bg]fsck is still needed, and it takes way too long on large 
(terabyte-size) file systems. This was not a problem with small file 
systems when SU was designed.
- bgfsck relies on snapshots, and there still are several problems with 
those (judging from information on mailing lists, not personal 
experience): they take too long to create on large file systems, there 
are problems with taking snapshots when the drive is almost full, and 
(possibly? don't remember exactly now) some NFS issues with either the 
snapshot or the underlying file system.
- (most?) kernel developers tend to disable bgfsck completely on the 
file systems actively used during development because of possible panic 
during testing new code while bgfsck is still running, and normal fsck 
takes too long
- todays desktop drives can lie about writing data. SoftUpdates relies 
on some assumptions about when the data is physically written to media, 
and those are not always valid today
- every now and then there appear some anecdotal evidence that file 
systems do get destroyed even with SoftUpdates, which are probably 
related to the previous point, but it can't be investigated
- AFAIK the addition of SoftUpdates to file system code made file system 
code more than usually complex, with bugs that are discovered even now.

The basic idea behind SU is good and valid, but judging from 
side-effects of its practical implementation, right now journaling 
appears to be a simpler and more effective solution to the problem.