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Re: [dm-devel] [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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