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

[Neil Brown]

On Mon, 29 Nov 2010 14:05:36 -0800 "Darrick J. Wong" <djwong at 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