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 -- 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
