Why is there no JFS?

Terry Lambert tlambert2 at mindspring.com
Fri Sep 24 08:53:29 PDT 2004

David Schultz wrote:
> > I think softupdates is still (viewed as) riskier than synchronous
> > writes, at least for large numbers of writes (like installworld) to a

NB: An initial system install is done with async mounts.  You can't
    use async mounts if you use soft updates, because the dependencies
    for already outstanding pending writes won't be there after a
    "mount -u".  Same is true of sync mounts, but install doesn't
    use that, or try to use "mount -u".

> I've heard that argument, and while I think it has *some* validity
> in general, I don't buy it for installworld/installkernel in
> particular.  Softupdates guarantees metadata consistency (modulo
> hardware issues that have been discussed on this list before), but
> it can reorder writes and delay the amount of time it takes your
> data to hit the disk.  For an installworld, this means that the
> window during which you have a partially installed world is
> slightly larger, but installworld takes a while, so the window is
> already pretty darn big.  The whole rationale for doing
> installworld/installkernel in a particular sequence is that with
> any luck, you can boot to single-user mode after something goes
> wrong and finish the job (or revert to the old kernel.)


This is the "Lightning is less likely to hit me if I play golf
very fast, even though I'm doing it in a thunderstorm" argument.

It's based on a false understanding of statistics, and it's the
same argument Linux FS people used to use, back before they had
an FS that ordered metadata writes, to justify not ordering
metadata writes (e.g. "use async, the failure window is smaller").

And we all know that's really bogus.  8-) 8-).

-- Terry

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