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    SubjectRE: Finding hardlinks
    On Sun, 2006-12-31 at 16:19 -0500, Halevy, Benny wrote:
    > Trond Myklebust wrote:
    > >
    > > On Thu, 2006-12-28 at 17:12 +0200, Benny Halevy wrote:
    > >
    > > > As an example, some file systems encode hint information into the filehandle
    > > > and the hints may change over time, another example is encoding parent
    > > > information into the filehandle and then handles representing hard links
    > > > to the same file from different directories will differ.
    > >
    > > Both these examples are bogus. Filehandle information should not change
    > > over time (except in the special case of NFSv4 "volatile filehandles")
    > > and they should definitely not encode parent directory information that
    > > can change over time (think rename()!).
    > >
    > > Cheers
    > > Trond
    > >
    > The first one is a real life example. Hints in the filehandle change
    > over time. The old filehandles are valid indefinitely, until the file
    > is deleted and hence can be considered permanent and not volatile.
    > What you say above, however, contradicts the NFS protocol as I understand
    > it. Here's some relevant text from the latest NFSv4.1 draft (the text in
    > 4.2.1 below exists in in a similar form also in NFSv3, rfc1813)
    > | 4.2.1. General Properties of a Filehandle
    > | ...
    > | If two filehandles from the same server are equal, they MUST refer to
    > | the same file. Servers SHOULD try to maintain a one-to-one correspondence
    > | between filehandles and files but this is not required. Clients MUST use
    > | filehandle comparisons only to improve performance, not for correct behavior
    > | All clients need to be prepared for situations in which it cannot be
    > | determined whether two filehandles denote the same object and in such cases,
    > | avoid making invalid assumptions which might cause incorrect behavior.
    > | Further discussion of filehandle and attribute comparison in the context of
    > | data caching is presented in the section "Data Caching and File Identity".
    > ...
    > | 9.3.4. Data Caching and File Identity
    > | ...
    > | When clients cache data, the file data needs to be organized according to
    > | the file system object to which the data belongs. For NFS version 3 clients,
    > | the typical practice has been to assume for the purpose of caching that
    > | distinct filehandles represent distinct file system objects. The client then
    > | has the choice to organize and maintain the data cache on this basis.
    > |
    > | In the NFS version 4 protocol, there is now the possibility to have
    > | significant deviations from a "one filehandle per object" model because a
    > | filehandle may be constructed on the basis of the object's pathname.
    > | Therefore, clients need a reliable method to determine if two filehandles
    > | designate the same file system object. If clients were simply to assume that
    > | all distinct filehandles denote distinct objects and proceed to do data
    > | caching on this basis, caching inconsistencies would arise between the
    > | distinct client side objects which mapped to the same server side object.
    > |
    > | By providing a method to differentiate filehandles, the NFS version 4
    > | protocol alleviates a potential functional regression in comparison with the
    > | NFS version 3 protocol. Without this method, caching inconsistencies within
    > | the same client could occur and this has not been present in previous
    > | versions of the NFS protocol. Note that it is possible to have such
    > | inconsistencies with applications executing on multiple clients but that is
    > | not the issue being addressed here.
    > |
    > | For the purposes of data caching, the following steps allow an NFS version 4
    > | client to determine whether two distinct filehandles denote the same server
    > | side object:
    > |
    > | * If GETATTR directed to two filehandles returns different values of the
    > | fsid attribute, then the filehandles represent distinct objects.
    > | * If GETATTR for any file with an fsid that matches the fsid of the two
    > | filehandles in question returns a unique_handles attribute with a value
    > | of TRUE, then the two objects are distinct.
    > | * If GETATTR directed to the two filehandles does not return the fileid
    > | attribute for both of the handles, then it cannot be determined whether
    > | the two objects are the same. Therefore, operations which depend on that
    > | knowledge (e.g. client side data caching) cannot be done reliably.
    > | * If GETATTR directed to the two filehandles returns different values for
    > | the fileid attribute, then they are distinct objects.
    > | * Otherwise they are the same object.

    Nobody promised you that NFSv4 would be consistent. The above is a crock
    of shit that carries over from RFC1813. fileid isn't even a mandatory
    attribute in NFSv4.

    > Even for NFSv3 (that doesn't have the unique_handles attribute I think
    > that the linux nfs client can do a better job. If you'd have a filehandle
    > cache that points at inodes you could maintain a many to one relationship
    > from multiple filehandles into one inode. When you discover a new filehandle
    > you can look up the inode cache for the same fileid and if one is found you
    > can do a getattr on the old filehandle (without loss of generality you should
    > always use the latest filehandle that was returned for that filesystem object,
    > although any filehandle that refers to it can be used).
    > If the getattr succeeded then the filehandles refer to the same fs object and
    > you can create a new entry in the filehandle cache pointing at that inode.
    > Otherwise, if getattr says that the old filehandle is stale I think you should
    > mark the inode as stale and keep it around so that applications can get an
    > appropriate error until last close, before you clean up the fh cache from the
    > stale filehandles. A new inode structure should be created for the new filehandle.

    No! Read what I said in that last email again. The above crap is
    precisely the algorithm I said is NOT wanted in the Linux client. We're
    NOT going to do a bunch of totally unnecessary extra GETATTR calls in
    order to cater to 1 or 2 servers out there that actually think the above
    is a good idea. Not even Solaris does that IIRC.


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