[Bug 251363] use unionfs as a disk-cache for NFS [feature]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=251363

--- Comment #19 from Gunther Schadow <raj@gusw.net> ---
I am certainly honored to be replied to by the one and only Kirk McKusick, and
of course I understand the cache consistency issue. HOWEVER, I am sure you will
also accept that the buffer cache is a very limited resource and generally too
small to hold a lot of binaries just in case.

The truth is that /bin, /usr/bin, /lib and all that good stuff hardly never
changes in a production system except for times of controlled upgrades.
Therefore the argument that the UNIONFS cache would become stale is not really
hard hitting.

I think my approach has a very good purpose when and where applied with an
understanding of the consequences.

Finally, if you really wanted to do an unlimited cache that is kept consistent,
then nothing should stop the addition of some additional update checks before
every access of the local cache on the source NFS, and likewise, writes could
be written through to the NFS backend. In other words, NFS could have a local
unlimited size disk cache feature. But I doubt anyone would have fulfilled my
wish, so I went ahead and provided small improvements to the UNIONFS code to
help me with my imperfect solution that's nevertheless good enough for me.

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