RE: Finding hardlinks

[Trond Myklebust <trond.myklebust@fys.uio.no>]

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.

Trond