On Thu, 2006-06-01 at 12:27 -0400, Xin Zhao wrote: > Question 1: ...and how many NFS implementations have you seen based on > that paper? > I don't know. I only read the NFS implementations distributed with > Linux kernel. But some paper mentioned that the soft update mechanism > suggested in that paper has been adopted by FreeBSD. FreeBSD does not use soft updates for NFS afaik. > Question 2: NFS permissions are checked by the _server_, not the client. > That's true. But I was not saying that all metadata access must be > asynchronous. Even for permission checking, speculative execution > mechanism proposed in Ed Nightingale's "speculative execution ...." > paper published in SOSP 2005 can be used to avoid waiting. The basic > idea is that a NFS client speculatively assume permission checking > returns "OK" and set a checkpoint, then the client can go ahead to > send further requests. If the actual result turns out to be "OK", the > client can discard the checkpoint, otherwise, it rolls back to the > checking point. This can make waiting time overlap with the sending > time of subsequent requests. ...and how does that help the user that has been told the operation succeeded? > Question 3: Cache consistency requirements are _much_ more stringent > for asynchronous operation. > I agree. But I am not sure how local file system like Ext3 handle this > problem. I don't think Ext3 must synchronously write metadata (I will > double check the ext3 code). If I remember correctly, when change > metadata, Ext3 just change it in memory and mark this page to be > dirty. The page will be flushed to disk afterward. If the server > exports an Ext3 code, it should be able to do the same thing. When a > client requests to change metadata, server writes to the mmaped > metadata page and then return to client instead of having to sync the > change to disk. With this mechanism, at least the client does not have > to wait for the disk flush time. Does it make sense? To prevent > interleave change on metadata before it is flushed to disk, the server > can even mark the metadata page to be read-only before it is flushed > to disk. 'man 5 exports'. Read _carefully_ the entry on the "async" export option, and see the NFS FAQ, nfs mailing list archives, etc... why it is a bad idea. 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
