Re: NFS dentry caching mechanism

[Peter Staubach <staubach@redhat.com>]

Trond Myklebust wrote:

>On Fri, 2006-01-27 at 08:38 -0500, Peter Staubach wrote:
>
>  
>
>>For systems which are based on the ONC+ code (Ie. Solaris), on write-able
>>file systems, the negative cache entries are _always_ validated using a
>>forced over the wire GETATTR operation.  Read-only file systems are
>>treated slightly differently by using the normal attribute cache mechanism
>>to do the validation.  This keeps the client from falling into this trap.
>>
>>It is okay for the client to think that a file exists which may not, because
>>it can detect the difference.  It is not okay for a client to decide that a
>>file does not exist without a strong validation mechanism because there is
>>no way for the application to determine otherwise.
>>    
>>
>
>That makes negative dentries more or less worthless: if you are going to
>force a GETATTR call every time, you might as well do a full lookup.
>
>  
>

Well, the need for the stronger consistency in this case reduces the
performance benefits, but does not eliminate them.  A GETATTR will
always be cheaper than a LOOKUP, especially one that will mostly
likely return ENOENT.

>We revalidate the parent directory (following the standard attribute
>caching rules - no forced GETATTR). If the parent directory has changed,
>we drop the negative dentry, and force a new lookup.
>

And this leads to the unacceptable problem that a correctly written
application may not work because of this cache.

Correctness first, then performance.

    Thanx...

       ps


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