On Wed, 2009-08-19 at 20:10 -0600, Andreas Dilger wrote: > On Aug 19, 2009 09:17 -0400, Josef Bacik wrote: > > > - does this guarantee also hold for remote filesystems (nfs / smb) ? > > > > This I'm more likely to be wrong on, but I don't think so. It would be atomic > > on the local machine, but if there is somebody else on another machine writing > > to the same file I think you would probably be screwed. > > With NFS at least, there is absolutely no guarantee of any kind when > multiple clients write to the same file, even with non-overlapping > writes (i.e. no O_APPEND, but application seeks to different file > offsets). That is because an NFS client does not necessarily flush > its local cache until it closes the file. That's not the main problem. NFS clients can (and do) flush writes immediately if you use O_DIRECT or synchronous writes. The real issue is rather that the NFS protocol does not support an atomic APPEND operation. Instead it requires the client to simulate append semantics, by first retrieving the file size (so that it can calculate the end-of-file offset) and then issuing the write. This means that races with other clients can always occur unless they are using some locking mechanism to prevent it. I believe the same is true of CIFS. Cheers Trond -- 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
