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    SubjectRe: NFS Client patch
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    >>>>> " " == Chris Wedgwood <cw@f00f.org> writes:

    > On Tue, Jul 10, 2001 at 10:22:16AM +0200, Trond Myklebust
    > wrote:
    > Imagine if somebody gives you a 1Gb directory. Would it or
    > would it not piss you off if your file pointer got reset to
    > 0 every time somebody created a file?

    > The current semantics are scalable. Anything which resets
    > the file pointer upon change of a file/directory/whatever
    > isn't...

    > Anyone using a 1GB directory deserves for it not to scale. I
    > think this is a very poor example.

    It's an extreme case, but it illustrates something the kernel should
    be able to cope with.

    > No that I disagree with you, the largest directories I have on
    > my system here are 2.6MB (freedb, lots of hashed flat-files in
    > one directory), here I do agree that you should not have to
    > reset the counter everytime.

    Right: this is what most people expect. The reason why the readdir
    code went through several quite different incarnations in the 2.3.x
    series was that duplicate directory entries were not acceptable to
    people.

    readdir() is not an atomic operation. You can't lock a directory while
    doing a series of readdir calls either on local filesystems nor over
    NFS. As such, the idea of volatile cookies doesn't really make sense,
    nor is it supported in rfc1094 (NFSv2):

    Each "entry" contains a "fileid" which consists of a unique number
    to identify the file within a filesystem, the "name" of the file,
    and a "cookie" which is an opaque pointer to the next entry in the
    directory. The cookie is used in the next READDIR call to get more
    entries starting at a given point in the directory.

    Nothing there states that the cookie can be invalidated, nor is there
    even an error to tell you that this is the case.


    In rfc1813 (NFSv3), they recognized that NFSv2 couldn't cope with
    stale cookies (yes: this fact is explicitly written down on pages 77
    and 78), and hence they introduced the cookie verifier and the
    NFS3ERR_BAD_COOKIE error, that can be used by the server to declare a
    cookie as being stale. In this case, some extra recovery action might
    make sense. I can see 3 possible solutions:

    1) The behaviour in this case is undefined. Leave it up to the
    user to reopen the directory, reset the file pointer, or whatever.
    This is what we do now.

    2) implement some extra caching info to allow an improved recovery
    of the last file position. This would likely have to involve
    storing the fileid + filename of the last entry somewhere in the
    struct file.

    This scheme means that lseek() breaks, and can undermine
    glibc. The latter has a lousy getdents algorithm in which it
    reads a number n of entries into a temporary buffer, the copy <=
    n entries to the user (because their struct dirent is larger than
    the kernel struct dirent), and then use lseek() to jump back.

    3) Implement something like Craig suggests whereby you reset the
    file pointer.

    This gives unexpected results as far as the user is concerned as
    it causes duplicate entries to pop up without any warning. It too
    breaks lseek(). It's a policy decision on behalf of the user.

    If someone can persuade the glibc people to implement a sane algorithm
    for getdents() that precalculates the upper limit on how much padding
    is needed, and drops the use of lseek(), then (2) might possibly be
    worth doing for 2.5.x (I believe I heard that Solaris does something
    along these lines).

    If not, given a choice between (1) and (3), I choose (1).



    Finally, any scheme that assumes cookie staleness in all cases where
    the directory mtime changes will not be passed on to Linus.

    Cheers,
    Trond
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