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From:   David Howells <dhowells@redhat.com>
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To:     bfields@fieldses.org (J. Bruce Fields)
Cc:     dhowells@redhat.com, Dave Wysochanski <dwysocha@redhat.com>,
        linux-nfs@vger.kernel.org, linux-cachefs@redhat.com,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Trond Myklebust <trondmy@hammerspace.com>
Subject: Re: [Linux-cachefs] [RFC PATCH v1 0/13] Convert NFS client to new fscache-iter API
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Date:   Fri, 17 Jul 2020 16:19:25 +0100
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J. Bruce Fields <bfields@fieldses.org> wrote:

> Say I had a hypothetical, err, friend, who hadn't been following that
> FS-Cache work--could you summarize the advantages it bring us?

https://lore.kernel.org/linux-nfs/159465784033.1376674.1810646369398981103=
7.stgit@warthog.procyon.org.uk/T/#t

 - Makes the caching code a lot simpler (~2400 LoC removed, ~1000 LoDoc[*]
   removed at the moment from fscache, cachefiles and afs).

 - Stops using bmap to work out what data is cached.  This isn't reliable =
with
   modern extend-based filesystems.  A bitmap of cached granules is saved =
in
   an xattr instead.

 - Uses async DIO (kiocbs) to do I/O to/from the cache rather than using
   buffered writes (kernel_write) and pagecache snooping for read (don't a=
sk).

   - A lot faster and less CPU intensive as there's no page-to-page copyin=
g.

   - A lot less VM pressure as it doesn't have duplicate pages in the back=
ing
     fs that aren't really accounted right.

 - Uses tmpfiles+link to better handle invalidation.  It will at some poin=
t
   hopefully employ linkat(AT_LINK_REPLACE) to effect cut-over on disk rat=
her
   than unlink,link.

David

[*] The upstream docs got ReSTified, so the doc patches I have are now use=
less
    and need reworking:-(.