On Tue, May 08, 2007 at 07:06:06AM -0400, Nick Holland wrote:
> George C wrote:
> > I've just stumbled across the SoftUpdates section in the FAQ, and was rather
> > surprised that I had never seen/heard of this feature before. Before
> > I mount any
> > partition using softdep, I thought I'd google, browse the archives, etc.
> > for any
> > information about when/where they should be used.
> >
> > Although I've found a plethora of information about soft updates, much of
> > it is
> > either contradictory or incomplete.... I thought I'd ask here for
> > clarification.
> >
> > Is it always best to mount /, /tmp, /usr, /var, /home with softdep?
> > Under what curcumstances would it not be appropriate?
>
> If your app makes assumptions about write ordering, softdeps can negate
> the care the app author took. For example, some mail programs don't ack
> the receipt of a message until it has been safely written to disk, the
> idea being that if the power goes out or the machine crashes, if the
> message has been acknowledged, IT HAS BEEN RECEIVED and will be there
> when the machine comes back up. Softdeps promises that what is on your
> disk is coherent, but "coherent" usually means the last few files written
> to disk may be just removed when the system comes back up. Not desired
> in this case.
this is not true. fsync() works as specified.
> Softdeps don't do anything for you if you are mostly reading from disk,
> or if the partition is mounted read-only. It's about writing.
of course they do. there are still atime updates
for example that will be handled if not mount read-only.
> Softdeps is much more complex than conventional disk access. While I
> have not personally seen a softdep-related bug in some time, and that
> one was quickly fixed, you HAVE to assume it is more likely to have
> bugs than the non-softdep systems.
this is also not exactly true -- there are softdep bugs fixed
at the rate of ten per year if not more. most of them are
bugs that been there forever.
cu
--
paranoic mickey (my employers have changed but, the name has remained)
