[Lustre-devel] subtree locks and path re-validation avoidance

From: Peter Braam <Peter.Braam@Sun.COM>
To: lustre-devel@lists.lustre.org
Subject: [Lustre-devel] subtree locks and path re-validation avoidance
Date: Sat, 01 Mar 2008 10:39:39 -0700	[thread overview]
Message-ID: <C3EEE2EB.28BE%peter.braam@sun.com> (raw)
In-Reply-To: <1204297512.24711.20.camel@linux.site>

Hi

On 2/29/08 8:05 AM, "Vladimir V. Saveliev" <Vladimir.Saveliev@Sun.COM>
wrote:

> Hello
> 
> On Fri, 2008-02-22 at 20:15 -0700, Peter J Braam wrote:
>> I'd like to make a suggestion to perhaps immediately find the right
>> primitives for getcwd to return a reasonably correct pathname in
>> Lustre.
> 
> Peter, would you say a bit more about that:
> 
> currently, there is nothing a filesystem can do in linux's getcwd. It
> simply returns instant dcache path from "." to "/".
> 

Yes.  So the question is what new dentry methods we might add so that the
dcache can call into the FS to validate the path.

The second question is then if these would be useful for revalidating
subtree lock paths.

- Peter -

>>   I believe this is the simplest case where I have seen pathname
>> revalidation being important.  In the context of that example the
>> subtree lock discussion might gain more clarity.
>> 
>> I would also like to note that I had a discussion with Linus at one of
>> the kernel workshops in Ottawa maybe almost 4-5 years ago.  First Linus
>> attacked the idea of using file identifiers - he suggested that doing
>> everything with pathnames was better (which is what InterMezzo did).
>> When we explained to him that this requires locking all parents he began
>> to see the problems we had with this and understood the locking at the
>> fid/name level that we use in Lustre.  I found little resistance when I
>> mentioned to him that for this model the VFS does not have a correct
>> implementation of getcwd, unless the dcache is kept current.
>> 
>> UCSC has received funding from the National Labs and now been turned
>> into a peta-scale I/O institute I believe did more results on file
>> systems implemented with pathnames.  Some things are beautiful and easy
>> with pathnames, but others get really ugly, and so far I don't see this
>> displacing fid ideas that govern NFS, AFS and Lustre.
>> 
> 
> Best regards,
> Vladimir
> 
> 
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