From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Xin Zhao" Subject: Re: Urgent help needed on an NFS question, please help!!! Date: Thu, 10 Aug 2006 13:08:45 -0400 Message-ID: <4ae3c140608101008t4e9a4451r8d1a7bd3c49c4f8b@mail.gmail.com> References: <4ae3c140608092204n1c07152k52010a10e209bb77@mail.gmail.com> <17626.49136.384370.284757@cse.unsw.edu.au> <4ae3c140608092254k62dce9at2e8cdcc9ae7a6d9f@mail.gmail.com> <17626.52269.828274.831029@cse.unsw.edu.au> <4ae3c140608100815p57c0378kfd316a482738ee83@mail.gmail.com> <20060810161107.GC4379@parisc-linux.org> <4ae3c140608100923j1ffb5bb5qa776bff79365874c@mail.gmail.com> <20060810165431.GD4379@parisc-linux.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: "Neil Brown" , linux-kernel , linux-fsdevel@vger.kernel.org Return-path: Received: from py-out-1112.google.com ([64.233.166.179]:2282 "EHLO py-out-1112.google.com") by vger.kernel.org with ESMTP id S1161465AbWHJRIq (ORCPT ); Thu, 10 Aug 2006 13:08:46 -0400 Received: by py-out-1112.google.com with SMTP id z74so908862pyg for ; Thu, 10 Aug 2006 10:08:45 -0700 (PDT) To: "Matthew Wilcox" In-Reply-To: <20060810165431.GD4379@parisc-linux.org> Content-Disposition: inline Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org Well. For regular NFS, because it needs to consider interoperability, it cannot use file handle as an opaque object. However, in our case, we essentially derived a VM based data sharing infrastructure from NFS. This would allow multiple virtual machines in a single server to share data efficiently. With some tricks, we are able to export inode cache from server to client. Also, we modify the file handle composer to carry the server-side inode address, inode number, i_gen, dev along with a file handle. Upon receiving a file handle, a client can directly access the inode object in the exported inode cache and bypass the inter-VM communication. So, in our case, we don't need to consider interoperability (at least for now), and we DO know the inode number, generation, as well as exported device info. I think this explains why I want to make sure the conclusion is right: Conclusion: Given a stored file handle and an inode object received from the server, an NFS client can safely determine whether this inode corresponds to the file handle by checking the inode+dev+i_generation. Many thanks for this helpful discussion. Xin On 8/10/06, Matthew Wilcox wrote: > On Thu, Aug 10, 2006 at 12:23:12PM -0400, Xin Zhao wrote: > > That makes sense. > > > > Can we make the following two conclusions? > > 1. In a single machine, inode+dev ID+i_generation can uniquely identify a > > file > > sure. > > > 2. Given a stored file handle and an inode object received from the > > server, an NFS client can safely determine whether this inode > > corresponds to the file handle by checking the inode+dev+i_generation. > > The NFS client makes up its own inode numbers for use on the local > machine. It doesn't know the device+inode+generation numbers on the > server (and indeed, the server may not even have the concepts of > inodes). To quote RFC 1813: > > The file handle contains all the information the server needs to > distinguish an individual file. To the client, the file handle is > opaque. The client stores file handles for use in a later request > and can compare two file handles from the same server for equality by > doing a byte-by-byte comparison, but cannot otherwise interpret the > contents of file handles. If two file handles from the same server > are equal, they must refer to the same file, but if they are not > equal, no conclusions can be drawn. Servers should try to maintain > a one-to-one correspondence between file handles and files, but this > is not required. Clients should use file handle comparisons only to > improve performance, not for correct behavior. >