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    SubjectRe: Why must NFS access metadata in synchronous mode?
    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

    > 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.


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