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

[Christoph Hellwig <hch lst de>]

On Tue, Nov 30, 2010 at 11:39:06AM +1100, Neil Brown wrote:
> 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.

We can even optimize the second flush away if no other I/O has
completed since the first flush has started.  That will always be the
case for SATA devices as the cache flush command is not queued.