On Fri, Feb 5, 2021, 7:19 AM Larry McVoy <lm@mcvoy.com> wrote:

> On Thu, Feb 04, 2021 at 09:17:54PM -0800, Bakul Shah wrote:
> > On Feb 4, 2021, at 4:33 PM, Larry McVoy <lm@mcvoy.com> wrote:
> > >
> > > Ignoring the page cache and make their own cache has big problems.
> > > You can mmap() ZFS files and doing so means that when a page is
> referenced
> > > it is copied from the ZFS cache to the page cache.  That creates a
> > > coherency problem, I can write via the mapping and I can write via
> > > write(2) and now you have two copies of the data that don't match,
> > > that's pretty much OS no-no #1.
> >
> > Write(2)ing to a mapped page sounds pretty dodgy. Likely to get you
> > in trouble in any case. Similarly read(2)ing.
>
> The entire point of the SunOS 4.0 VM system was that the page you
> saw via mmap(2) is the exact same page you saw via read(2).  It's
> the page cache, it has page sized chunks of memory that cache
> file,offset pairs.
>
> There is one, and only one, copy of the truth.  Doesn't matter how
> you get at it, there is only one "it".
>
> ZFS broke that contract and that was a step backwards in terms of
> OS design.
>

The double copy is the primary reason we don't use it to store videos we
serve. It's a performance bottleneck as well.

And fixing it is... rather involved... possible, but a lot of work to teach
the ARC about the buffer cache or the buffer cache about the ARC...

But for everything else I do, I accept the imperfect design because of all
the other features it unlocks.

Warner

>