Here is the copy from https://tools.ietf.org/html/rfc7530#page-40
Will Client query 'change' attribute every time before reading to know
if the data has been changed?

      +-----------------+----+------------+-----+-------------------+
      | Name            | ID | Data Type  | Acc | Defined in        |
      +-----------------+----+------------+-----+-------------------+
      | supported_attrs | 0  | bitmap4    | R   | Section 5.8.1.1   |
      | type            | 1  | nfs_ftype4 | R   | Section 5.8.1.2   |
      | fh_expire_type  | 2  | uint32_t   | R   | Section 5.8.1.3   |
      | change          | 3  | changeid4  | R   | Section 5.8.1.4   |
      | size            | 4  | uint64_t   | R W | Section 5.8.1.5   |
      | link_support    | 5  | bool       | R   | Section 5.8.1.6   |
      | symlink_support | 6  | bool       | R   | Section 5.8.1.7   |
      | named_attr      | 7  | bool       | R   | Section 5.8.1.8   |
      | fsid            | 8  | fsid4      | R   | Section 5.8.1.9   |
      | unique_handles  | 9  | bool       | R   | Section 5.8.1.10  |
      | lease_time      | 10 | nfs_lease4 | R   | Section 5.8.1.11  |
      | rdattr_error    | 11 | nfsstat4   | R   | Section 5.8.1.12  |
      | filehandle      | 19 | nfs_fh4    | R   | Section 5.8.1.13  |
      +-----------------+----+------------+-----+-------------------+

On Thu, Feb 14, 2019 at 8:29 PM Jeff Layton <jlay...@poochiereds.net> wrote:
>
> On Thu, 2019-02-14 at 19:49 +0800, Marvin Zhang wrote:
> > Hi Jeff,
> > Another question is about Client Caching when disabling delegation.
> > I set breakpoint on nfs4_op_read, which is OP_READ process function in
> > nfs-ganesha. Then I read a file, I found that it will hit only once on
> > the first time, which means latter reading operation on this file will
> > not trigger OP_READ. It will read the data from client side cache. Is
> > it right?
>
> Yes. In the absence of a delegation, the client will periodically query
> for the inode attributes, and will serve reads from the cache if it
> looks like the file hasn't changed.
>
> > I also checked the nfs client code in linux kernel. Only
> > cache_validity is NFS_INO_INVALID_DATA, it will send OP_READ again,
> > like this:
> >     if (nfsi->cache_validity & NFS_INO_INVALID_DATA) {
> >         ret = nfs_invalidate_mapping(inode, mapping);
> >     }
> > This about this senario, client1 connect ganesha1 and client2 connect
> > ganesha2. I read /1.txt on client1 and client1 will cache the data.
> > Then I modify this file on client2. At that time, how client1 know the
> > file is modifed and how it will add NFS_INO_INVALID_DATA into
> > cache_validity?
>
>
> Once you modify the code on client2, ganesha2 will request the necessary
> caps from the ceph MDS, and client1 will have its caps revoked. It'll
> then make the change.
>
> When client1 reads again it will issue a GETATTR against the file [1].
> ganesha1 will then request caps to do the getattr, which will end up
> revoking ganesha2's caps. client1 will then see the change in attributes
> (the change attribute and mtime, most likely) and will invalidate the
> mapping, causing it do reissue a READ on the wire.
>
> [1]: There may be a window of time after you change the file on client2
> where client1 doesn't see it. That's due to the fact that inode
> attributes on the client are only revalidated after a timeout. You may
> want to read over the DATA AND METADATA COHERENCE section of nfs(5) to
> make sure you understand how the NFS client validates its caches.
>
> Cheers,
> --
> Jeff Layton <jlay...@poochiereds.net>
>
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