On Wed, 2011-01-05 at 21:01 +0200, Benny Halevy wrote: > On 2011-01-03 16:40, Trond Myklebust wrote: > > On Mon, 2011-01-03 at 16:21 +0200, Benny Halevy wrote: > >> On 2010-12-17 01:07, Christoph Hellwig wrote: > >>> On Thu, Dec 16, 2010 at 11:21:21AM -0500, Matt W. Benjamin wrote: > >>>> Hi, > >>>> > >>>> We have a files implementation which wants to receive LAYOUTCOMMIT when a client is finished with a layout. It was my clear understanding from rfc5661 that we could expect this behavior. > >>> > >>> Care to post it to the list? > >>> > >> > >> I don't know what Matt's server is doing but the fundamental problem is > >> manifested with extending a file with parallel DS writes. > >> Assuming that the DS writes are executed in arbitrary order, > >> exposing the file length before LAYOUTCOMMIT can cause > >> a concurrent reader to read a hole. Although locking can > >> solve this case, day-to-day applications that work well over > >> local filesystem and legacy NFS may break because of this. > > > > ...and this differs from ordinary NFS writes exactly how? > > > > Both cached and uncached (i.e. O_DIRECT) writes can and will be flushed > > to disk in entirely random order when writing to the MDS. If you have a > > parallel reader on another client (or even on the same client in the > > case of O_DIRECT), and want it to see accurate data, then use locking. > > If not, you will see holes and other strangeness. > > > > IOW: There are no 'day-to-day applications that work well over legacy > > NFS' that rely on this behaviour. > > > > Assuming the client writes sequentially (over tcp) the writes will > practically be processed in order into the server's cache so with > no crashes in the mix a parallel reader will see no holes. > I'd really like the following scenario to work over pNFS with > no hassles: > "some app >> foo" on one client, and > "tail -f foo" on another No, that doesn't work today! Believe me, I get the "bug reports"... There is no point in trying to add properties to pNFS that don't exist with ordinary NFS. Trond -- Trond Myklebust Linux NFS client maintainer NetApp Trond.Myklebust@xxxxxxxxxx www.netapp.com -- 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
