On Wed, Nov 14, 2007 at 05:18:50PM -0500, Trond Myklebust wrote: > > On Wed, 2007-11-14 at 22:50 +0100, Peter Zijlstra wrote: > > Right, but I guess what Nick asked is, if pages could be stale to start > > with, how is that avoided in the future. > > > > The way I understand it, this re-validate is just a best effort at > > getting a coherent image. > > The normal convention for NFS is to use a close-to-open cache > consistency model. In that model, applications must agree never to open > the file for reading or writing if an application on a different NFS > client already holds it open for writing. > > However there is no standard locking model for _enforcing_ such an > agreement, so some setups do violate it. One obvious model that we try > to support is that where the applications are using POSIX locking in > order to ensure exclusive access to the data when requires. > > Another model is to rely rather on synchronous writes and heavy > attribute revalidation to detect when a competing application has > written to the file (the 'noac' mount option). While such a model is > obviously deficient in that it can never guarantee cache coherency, we > do attempt to ensure that it works on a per-operation basis (IOW: we > check cache coherency before each call to read(), to mmap(), etc) since > it is by far the easiest model to apply if you have applications that > cannot be rewritten and that satisfy the requirement that they rarely > conflict. mmap()s can be different from read in that the syscall may have little relation to when the data gets used. But I guess it's still a best effort thing. Fair enough. Thanks, Nick - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
