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    SubjectRe: [PATCH v6 0/4] ext4: Coordinate data-only flush requests sent by fsync
    On 11/29/2010 07:39 PM, Neil Brown wrote:
    > On Mon, 29 Nov 2010 14:05:36 -0800 "Darrick J. Wong"<djwong@us.ibm.com>
    > wrote:
    >
    >> On certain types of hardware, issuing a write cache flush takes a considerable
    >> amount of time. Typically, these are simple storage systems with write cache
    >> enabled and no battery to save that cache after a power failure. When we
    >> encounter a system with many I/O threads that write data and then call fsync
    >> after more transactions accumulate, ext4_sync_file performs a data-only flush,
    >> the performance of which is suboptimal because each of those threads issues its
    >> own flush command to the drive instead of trying to coordinate the flush,
    >> thereby wasting execution time.
    >>
    >> Instead of each fsync call initiating its own flush, there's now a flag to
    >> indicate if (0) no flushes are ongoing, (1) we're delaying a short time to
    >> collect other fsync threads, or (2) we're actually in-progress on a flush.
    >>
    >> So, if someone calls ext4_sync_file and no flushes are in progress, the flag
    >> shifts from 0->1 and the thread delays for a short time to see if there are any
    >> other threads that are close behind in ext4_sync_file. After that wait, the
    >> state transitions to 2 and the flush is issued. Once that's done, the state
    >> goes back to 0 and a completion is signalled.
    > I haven't seen any of the preceding discussion do I might be missing
    > something important, but this seems needlessly complex and intrusive.
    > In particular, I don't like adding code to md to propagate these timings up
    > to the fs, and I don't the arbitrary '2ms' number.
    >
    > Would it not be sufficient to simply gather flushes while a flush is pending.
    > i.e
    > - if no flush is pending, set the 'flush pending' flag, submit a flush,
    > then clear the flag.
    > - if a flush is pending, then wait for it to complete, and then submit a
    > single flush on behalf of all pending flushes.
    >
    > That way when flush is fast, you do a flush every time, and when it is slow
    > you gather multiple flushes together.
    > I think it would issues a few more flushes than your scheme, but it would be
    > a much neater solution. Have you tried that and found it to be insufficient?
    >
    > Thanks,
    > NeilBrown
    >

    The problem with that is that you can introduce a wait for the next flush longer
    than it would take to complete the flush. Having the wait adjust itself
    according to the speed of the device is much better I think....

    Ric



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