On Mon, 2009-11-09 at 08:25 -0500, Trond Myklebust wrote: > On Mon, 2009-11-09 at 14:42 +0200, Ivan Yosifov wrote: > > Hi everyone, > > > > I'm new to NFS and am trying to understand how to use it safely and > > correctly. > > > > The situation I'm considering is following. There's a server with an > > ( ext4 if it matters ) local volume that's exported through NFSv4. There > > are expected to be both remote ( ie. over the NFS ) read/write clients > > and local ones that write to the volume directly ( ie. not thorough the > > NFS ). > > > > Assuming well-behaved clients, ie. no interleaved open()...close() on > > the same file, how will NFS handle this ? Will close-to-open consistency > > continue to work, will it interfere with delegations ? > > > > Pointers or thoughts greatly appreciated. > > The NFS expectation is that if one application is writing to a file > while another is holding it open, then the two applications will use > locking in order to synchronise their access. So I understood. So far so good. > As long as all applications adhere to this rule, then you are supposed > to be guaranteed cache consistency. It shouldn't matter if the > applications are running on the server or on another client. > Cheers > Trond > Sounds encouraging. Just to clarify, I assume local writers don't pass through NFS. Out of curiosity, do you know how it's implemented ? I looked at fs/nfsd and fs/nfs in the kernel source but didn't really understand a lot. I got the impression that nfs clients and the server pass nfsd4_change_info objects ( defined in include/linux/nfsd/xdr4.h ) around with the before_change and after_change fields being "file version" of sorts - ie. they are changed when the file is changed and don't depend on timestamp resolution or anything like that. My concern is that the normal fs has only timestamps ( right ? ) which have a finite resolution and can't be used as file version indicators. So it seems necessary for the normal fs code to notify the NFS code for every write/change to a file that's also opened through NFS. Such a tight coupling both seems hard to get and I didn't notice anything like it in the source. Thanks, Ivan -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
