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    SubjectRe: Nanosecond fs timestamp support: sad
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    On Fri, Jul 22, 2011 at 11:47:32PM +0200, Andi Kleen wrote:
    > On Fri, Jul 22, 2011 at 04:11:42PM -0500, Matt Mackall wrote:
    > > On Fri, 2011-07-22 at 22:59 +0200, Andi Kleen wrote:
    > > > > Indeed. Only usefully exists on ext4 and requires extra system calls.
    > > >
    > > > Not sure what you mean? It's in stat(2), just like the timestamps.
    > >
    > > I don't see anything that looks like a version or generation number in
    > > either the man pages, the asm-generic/stat.h, or glibc's asm/stat.h.
    > > Pointer?
    >
    > Hmm you're right. I thought it was in there, but apparently not.
    > I think it should be added there though. We still have some unused
    > fields.

    But last I checked I thought it was only ext4 that actually incremented
    the i_version on IO, and even then only when given a (non-default) mount
    option.

    My notes on what needs to be done there:

    - collect data to determine whether turning on i_version causes
    any significant performance regressions.
    - Last I talked to him, Ted Tso recommended running
    Bonnie on a local disk, since it does a lot of little
    writes, which is somewhat of a worst case, as it will
    generate extra metadata updates for each write.
    Compare total wall-clock time, number of iops, and
    number of bytes (using some kind of block tracing).
    - If there aren't any problems, turn it on by default, and we're
    done. If there are unfixable problems, consider something
    more complicated (like turning on i_version automatically when
    someone asks for it).
    - We need to check that i_version is also doing something
    sensible on directory as well as on file inodes.
    - We also need to think about what it does after reboots. (E.g.
    what is an nfs server to do if clients see the i_version go
    backwards (and hence possible repeat old values) after a
    reboot?)
    - Double-check the order that data updates and i_version updates
    are done in. (Ideal would be if they were atomic, but for
    nfsd's purposes at least it should be adequate if the
    i_version comes after, and no later than the next commit.)

    --b.


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