linux-kernel - Re: [PATCH 00/67] fscache: Rewrite index API and management system

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Message-ID: <2972094.1634670049@warthog.procyon.org.uk>
Date:   Tue, 19 Oct 2021 20:00:49 +0100
From:   David Howells <dhowells@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com,
        ceph-devel@...r.kernel.org, linux-afs@...ts.infradead.org,
        Anna Schumaker <anna.schumaker@...app.com>,
        linux-nfs@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-mm@...ck.org, Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org,
        Dave Wysochanski <dwysocha@...hat.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Shyam Prasad N <nspmangalore@...il.com>,
        Eric Van Hensbergen <ericvh@...il.com>,
        v9fs-developer@...ts.sourceforge.net, linux-cifs@...r.kernel.org,
        Latchesar Ionkov <lucho@...kov.net>,
        Steve French <sfrench@...ba.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Trond Myklebust <trondmy@...merspace.com>,
        Omar Sandoval <osandov@...ndov.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/67] fscache: Rewrite index API and management system

Jeff Layton <jlayton@...nel.org> wrote:

> Given the indexing changes, what sort of behavior should we expect when
> upgrading from old-style to new-style indexes? Do they just not match,
> and we end up downloading new copies of all the data and the old stale
> stuff eventually gets culled?

Correct: they don't match.  The names of the directories and files will be
quite different - and so will the attached xattrs.  However, no filesystems
currently store locally-modified data in the cache, so you shouldn't lose any
data after upgrading.

> Ditto for downgrades -- can we expect sane behavior if someone runs an
> old kernel on top of an existing fscache that was populated by a new
> kernel?

Correct.  With this branch, filesystems now store locally-modified data into
the cache - but they also upload it to the server at the same time.  If
there's a disagreement between what's in the cache and what's on the server
with this branch, the cache is discarded, so simply discarding the cache on a
download shouldn't be a problem.

It's currently operating as a write-through cache, not a write-back cache.
That will change if I get round to implementing disconnected operation, but
it's not there yet.

David