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

From: <bugzilla-noreply_at_freebsd.org>
Date: Wed, 19 May 2021 04:52:35 UTC

--- Comment #20 from Kirk McKusick <mckusick@FreeBSD.org> ---
(In reply to Gunther Schadow from comment #19)
The buffer cache is no longer a limited resource as it has become merged with
the VM cache. Actually, the buffer cache is just a compatibility shim over the
VM cache. The point is that the entire physical memory is managed by the VM
cache. Most of the pages are associated with pages in filesystems and some are
associated with running programs (stacks and heap pages). So whether you use
those pages as part of NFS or the memory filesystem aggregated through the
union filesystem, they are all coming from the same pool. Given the broken
semantics and extra overhead of the union filesystem, you are better off
letting NFS rather than the union and memory filesystem manage the pages.

The role of the union filesystem is to allow the appearance of modification of
read-only media like DVDs or CD-ROMs.

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