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From:   David Howells <dhowells@redhat.com>
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To:     Jeff Layton <jlayton@kernel.org>
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Subject: Re: [PATCH 00/67] fscache: Rewrite index API and management system
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Date:   Tue, 19 Oct 2021 20:00:49 +0100
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Jeff Layton <jlayton@kernel.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