Re: [PATCH] nfs(5): Treatment of *atime mount options

[Steve Dickson <SteveD@redhat.com>]

On 18/11/13 10:36, Chuck Lever wrote:
> I was reminded recently that NFS treats file atime time stamps
> differently than other filesystems.  It also ignores the generic
> *atime mount options because it cannot support the atime semantics
> of local filesystems.
> 
> We should document that somewhere.  nfs(5) seems like a logical
> place for it.
> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Committed (tag: nfs-utils-1-2-10-rc1)

steved.

> ---
>  utils/mount/nfs.man |   59 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 59 insertions(+)
> 
> diff --git a/utils/mount/nfs.man b/utils/mount/nfs.man
> index 67031b5..2250963 100644
> --- a/utils/mount/nfs.man
> +++ b/utils/mount/nfs.man
> @@ -1227,6 +1227,65 @@ If absolute cache coherence among clients is required,
>  applications should use file locking. Alternatively, applications
>  can also open their files with the O_DIRECT flag
>  to disable data caching entirely.
> +.SS "File timestamp maintainence"
> +NFS servers are responsible for managing file and directory timestamps
> +.RB ( atime ,
> +.BR ctime ", and"
> +.BR mtime ).
> +When a file is accessed or updated on an NFS server,
> +the file's timestamps are updated just like they would be on a filesystem
> +local to an application.
> +.P
> +NFS clients cache file attributes, including timestamps.
> +A file's timestamps are updated on NFS clients when its attributes
> +are retrieved from the NFS server.
> +Thus there may be some delay before timestamp updates
> +on an NFS server appear to applications on NFS clients.
> +.P
> +To comply with the POSIX filesystem standard, the Linux NFS client
> +relies on NFS servers to keep a file's
> +.B mtime
> +and
> +.B ctime
> +timestamps properly up to date.
> +It does this by flushing local data changes to the server
> +before reporting
> +.B mtime
> +to applications via system calls such as
> +.BR stat (2).
> +.P
> +The Linux client handles
> +.B atime
> +updates more loosely, however.
> +NFS clients maintain good performance by caching data,
> +but that means that application reads, which normally update
> +.BR atime ,
> +are not reflected to the server where a file's
> +.B atime
> +is actually maintained.
> +.P
> +Because of this caching behavior,
> +the Linux NFS client does not support generic atime-related mount options.
> +See
> +.BR mount (8)
> +for details on these options.
> +.P
> +In particular, the
> +.BR atime / noatime ,
> +.BR diratime / nodiratime ,
> +.BR relatime / norelatime ,
> +and
> +.BR strictatime / nostrictatime
> +mount options have no effect on NFS mounts.
> +.P
> +.I /proc/mounts
> +may report that the
> +.B relatime
> +mount option is set on NFS mounts, but in fact the
> +.B atime
> +semantics are always as described here, and are not like
> +.B relatime
> +semantics.
>  .SS "Directory entry caching"
>  The Linux NFS client caches the result of all NFS LOOKUP requests.
>  If the requested directory entry exists on the server,
>