Re: [PATCH 0/3] Intro: Better handling coarse-grained timestamps

[Bob Bell <b_linuxnfs-Y/+76LoPTq9wBoktGHYdvgC/G2K4zDHf@public.gmane.org>]

On Tue, Jan 15, 2008 at 11:27:01AM -0500, Bob Bell wrote:
>Secondly, a modification to the kernel has been made to disable the 
>caching of negative dentries when the NFS_MOUNT_NONEGDE flag is set.  
>Note that this does nothing to address outdated information being 
>returned by readdir().  However, it does allow a file to be open()ed or 
>lstat()ed if the filename is known.  Because this has a potential 
>performance impact, it is left as an option for the user to select.

Someone pointed me to this page on NFSv4.1 directory delegations: 
http://wiki.linux-nfs.org/wiki/index.php/NFSv4.1_Directory_Delegations

I found the following quotes intriguing:

"Even though the directories along the path are cached, [without 
directory delegations] negative dentry caching is not allowed because it 
potentially violates close-to-open consistency semantics."

"Close-to-open consistency currently requires that even in a case where 
previous LOOKUPs or OPENs for a given file have recently and repeatedly 
failed, subsequent LOOKUPs and OPENs must nevertheless be sent to the 
server (i.e., negative caching provides no benefit in those cases)."

I admittedly know little about what is being discussed here.  However, 
is it possible that the Linux NFS client should *NEVER* cache negative 
entries?  (well, maybe with the exception of "nocto")

I'm interested if anyone has thoughts on the matter, though I know don't 
enough to argue either side, so here's a really easy chance to win an 
argument. :)

-- 
Bob Bell