Re: [TUHS] FreeBSD behind the times? (was: Favorite unix design principles?)

[Dan Cross <crossd@gmail.com>]

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On Thu, Feb 4, 2021 at 10:47 AM Will Senn <will.senn@gmail.com> wrote:

> [snip]
>
> In response to the negative vibes around ZFS. [snip]
>

I think the discordance is around the semantics ZFS's implementation
implies. Larry's point about mmap() vs a buffer cache is entirely valid; it
took lots of people heroic amounts of work worthy of Greek sagas to bridge
the difference between the original buffer and VM page caches, but ZFS
says, "meh. too much work; not worth it." The practical implication of that
is that memory mapped IO (via `mmap`) is no longer coherent with file IO
(via `open`/`close`/`read`/`write`) without lots of work that both degrades
performance and add complexity.

The question that a lot of folks who use ZFS regularly ask is, "does that
matter?" And perhaps it doesn't: if I've got a file server sitting there
serving NFS, do I care what it's kernel is doing? As long as it's
saturating the network and disks, and it's reliable...not really.
(Incidentally, that was kind of the philosophy behind the original plan9
file server kernel...as I heard the story, the rate of change of the plan9
kernel proper was too high, so Ken split off the file server portion into
its own, special-purpose kernel, and it stayed like that for ~20 years).
Similarly, if I'm on the local machine and the required coherence code is
there and largely works, then again, perhaps as a consumer of the
filesystem, I just don't care. After all, one can still get work done, and
ZFS has a bunch of other features that make it very attractive, right? In
particular, it's very good at NOT losing my data, kernel purity be damned.

On the other hand, if we're discussing OS design and implementation,
(re)splitting the VM and buffer caches is a poor decision. One might well
ask, "why?" and the answer may be, "because it adds significant complexity
to the kernel." This to me seems like the crux of the disagreement.
Satisfied users of ZFS might legitimately ask, "who cares?" and one might
respond, "kernel maintainers." If the kernel is mostly transparent as far
as a particular use case goes, though, then I can see why one would bulk at
the suggestion that this matters. If one is concerned with the design and
implementation of kernels, I could see why one would care very much.

Like many things, it's a matter of perspective.

        - Dan C.

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