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    SubjectRe: [PATCH v6 0/4] ext4: Coordinate data-only flush requests sent by fsync
    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





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