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

From: <bugzilla-noreply_at_freebsd.org>
Date: Tue, 18 May 2021 20:46:33 UTC

Kirk McKusick <mckusick@FreeBSD.org> changed:

           What    |Removed                     |Added
             Status|New                         |Open
                 CC|                            |mckusick@FreeBSD.org

--- Comment #18 from Kirk McKusick <mckusick@FreeBSD.org> ---
Using the union filesystem to cache NFS is a bad idea. The whole point of NFS
is to maintain cache coherency between the clients and the server. If a client
writes a file, NFS will ensure that all the other NFS clients can see those
changes. If a new copy of a binary is installed on the server, NFS will ensure
that all the clients see the updated binary. The union filesystem provides no
coherency at all.

NFS already does caching using the buffer cache. The buffer cache is a better
way to cache binaries as it only holds the pages of the binary that you are
actively using. By contrast unionfs is going to copy the entire binary image
before it can start up. And it will hold all of the binary in memory even if
only a small part of it is being used.

This technique may be usable in your environment, but it is absolutely not
useful as a general-purpose tool.

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