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    SubjectRE: Pull request for FS-Cache, including NFS patches
    Before throwing the 'FUD' acronym around, maybe you should re-read the
    details. My point was that there were few users of cachefs even when
    the technology had the potential for greater benefit (slower networks,
    less powerful servers, smaller memory caches). Obviously cachefs can
    improve performance--it's simply a function of workload and the
    assumptions made about server/disk/network bandwidth. However, I would
    expect the real benefits and real beneficiaries to be fewer than in the
    past. HOWEVER^2 I did provide some argument(s) in favor of adding
    cachefs, and look forward to extensions to support delayed write,
    offline operation, and NFSv4 support with real consistency checking (as
    long as I don't have to take the customer calls ;-). BTW,
    animation/video shops were one group that did benefit, and I imagine
    they still could today (the one I had in mind did work across Britain,
    the US, and Asia and relied on cachefs for overcoming slow network
    connections). Wonder if the same company is a RH customer...

    All the comparisons to HTTP browser implementations are, imho, absurd.
    It's fine to keep a bunch of http data around on disk because a) it's RO
    data, b) correctness is not terribly important, and c) a human is
    generally the consumer and can manually request non-cached data if
    things look wonky. It is a trivial case of caching.

    As for security, look at what MIT had to do to prevent local disk
    caching from breaking the security guarantees of AFS.

    Customers (deluded or otherwise) are still customers. No one is forced
    to compile it into their kernel. Ship it.


    -----Original Message-----
    From: Trond Myklebust []
    Sent: Monday, December 29, 2008 6:31 AM
    To: Andrew Morton
    Cc: Stephen Rothwell; Bernd Schubert;;;;;;;
    Subject: Re: Pull request for FS-Cache, including NFS patches

    On Sun, 2008-12-28 at 20:01 -0800, Andrew Morton wrote:
    > On Mon, 29 Dec 2008 14:45:33 +1100 Stephen Rothwell
    <> wrote:
    > > Hi David,
    > >
    > > On Fri, 19 Dec 2008 11:05:39 +1100 Stephen Rothwell
    <> wrote:
    > > >
    > > > Given the ongoing discussions around FS-Cache, I have removed it
    > > > from linux-next. Please ask me to include it again (if sensible)
    > > > once some decision has been reached about its future.
    > >
    > > What was the result of discussions around FS-Cache?
    > There was none.
    > Dan Muntz's question:
    > Solaris has had CacheFS since ~1995, HPUX had a port of it since
    > ~1997. I'd be interested in evidence of even a small fraction of
    > Solaris and/or HPUX shops using CacheFS. I am aware of customers
    > thought it sounded like a good idea, but ended up ditching it for
    > various reasons (e.g., CacheFS just adds overhead if you almost
    > always hit your local mem cache).
    > was an very very good one.
    > Seems that instead of answering it, we've decided to investigate the
    > fate of those who do not learn from history.

    David has given you plenty of arguments for why it helps scale the
    server (including specific workloads), has given you numbers validating
    his claim, and has presented claims that Red Hat has customers using
    cachefs in RHEL-5.
    The arguments I've seen against it, have so far been:

    1. Solaris couldn't sell their implementation
    2. It's too big
    3. It's intrusive

    Argument (1) has so far appeared to be pure FUD. In order to discuss the
    lessons of history, you need to first do the work of analysing and
    understanding it first. I really don't see how it is relevant to Linux
    whether or not the Solaris and HPUX cachefs implementations worked out
    unless you can demonstrate that that their experience shows some fatal
    flaw in the arguments and numbers that David presented, and that his
    customers are deluded.
    If you want examples of permanent caches that clearly do help servers
    scale, then look no further than the on-disk caches used in almost all
    http browser implemantations. Alternatively, as David mentioned, there
    are the on-disk caches used by AFS/DFS/coda.

    (2) may be valid, but I have yet to see specifics for where you'd like
    to see the cachefs code slimmed down. Did I miss them?

    (3) was certainly true 3 years ago, when the code was first presented
    for review, and so we did a review and critique then. The NFS specific
    changes have improved greatly as a result, and as far as I know, the
    security folks are happy too. If you're not happy with the parts that
    affect the memory management code then, again, it would be useful to see
    specifics that what you want changed.

    If there is still controversy concerning this, then I can temporarily
    remove cachefs from the nfs linux-next branch, but I'm definitely
    keeping it in the linux-mm branch until someone gives me a reason for
    why it shouldn't be merged in its current state.


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