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    SubjectRe: Nanosecond fs timestamp support: sad
    On Sat, Jul 23, 2011 at 08:59:15AM +1000, NeilBrown wrote:
    > On Fri, 22 Jul 2011 18:31:58 -0400 "J. Bruce Fields" <bfields@fieldses.org>
    > wrote:
    >
    > > On Fri, Jul 22, 2011 at 06:10:39PM -0400, bfields wrote:
    > > > 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.
    > >
    > > (Well,and talk the other filesystem implementors into doing it.)
    > >
    >
    > But does anyone apart from NFSv4 actually *want* i_version as opposed to the
    > more-generally-useful precise timestamps?

    It *seems* like a generally useful idea, but I don't know of any other
    users.

    > If not, we probably should tell NFSv4 to use timestamps and focus on making
    > them work well.
    > ??

    Well, sure, I couldn't complain about that if that proved possible.

    --b.

    >
    > The timestamp used doesn't need to update ever nanosecond. I think if it
    > were just updated on every userspace->kernel transition (or effective
    > equivalents inside kernel threads) that would be enough capture all
    > causality. I wonder how that would be achieved.. I wonder if RCU machinery
    > could help - doesn't it keep track of when threads schedule ... or something?
    >
    > NeilBrown


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