gjournal and TRIM: A safe combination?

Julien Cigar jcigar at ulb.ac.be
Wed Oct 21 14:36:42 UTC 2015

On Wed, Oct 21, 2015 at 09:15:33AM -0500, Adam Vande More wrote:
> On Wed, Oct 21, 2015 at 8:44 AM, Brandon J. Wandersee <
> brandon.wandersee at gmail.com> wrote:
> > I thought the opposite, myself. `gjournal` has its own section in the
> > Handbook, while SU+J isn't even mentioned.
> Unfortunately, this is not a safe assumption.  The handbook is maintained
> by volunteers.  The volunteers generally do a pretty good job, but are a
> few negatives of this approach.  They aren't generally interested in
> updating old documentation which doesn't directly apply to them.  The
> provided documentation is how they think it should be done: right, wrong or
> somewhere in-between.
> A lot of things don't get the attention they deserve like SU+J, GELI over
> GBDE, and in-kernel NAT w/ FreeBSD single native firewall ipfw.
> > I also thought there was
> > something fundamentally different about them--like SU+J simply maintains
> > filesystem integrity, while gjournal actually protects files from the
> > write hole as well as protecting the filesystem. I could be wrong.
> >
> There is a fundamental difference between them.  Don't you think you should
> investigate the differences prior to deciding what to use?
> It's not at clear what your goal is here.  Do you want to minimize writes
> to the ssd or do you want to ensure the fs is always consistent from both a
> meta-data and data perspective.  These options are mutually exclusive in
> the parameters you've laid out.

to resume:

SU = metadata consistency
SU+J = journaled metadata consistency 
gjournal = metadata + data consistency (block based)

Correct me if I'm wrong but, unless there is a bug in Soft Updates (SU),
your FS will, in theory, never be corrupted with SU only
The main advantage of SU+J over SU is to avoid a fsck at boot if the FS 
is not clean. Note that SU+J almost never worked for me and disabling
SU+J (tunefs -j disable) is the first thing I do after an installation.

> -- 
> Adam
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Julien Cigar
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