Re: [PATCH 3/3] ext4: update writeback_index based on last page scanned

[Eric Sandeen <sandeen@redhat.com>]

Ted Ts'o wrote:
> On Fri, Oct 22, 2010 at 04:45:17PM -0500, Eric Sandeen wrote:
>> As pointed out in a prior patch, updating the mapping's
>> writeback_index based on pages written isn't quite right;
>> what the writeback index is really supposed to reflect is
>> the next page which should be scanned for writeback during
>> periodic flush.
>>
>> As in write_cache_pages(), write_cache_pages_da() does
>> this scanning for us as we assemble the mpd for later
>> writeout.  If we keep track of the next page after the
>> current scan, we can easily update writeback_index without
>> worrying about pages written vs. pages skipped, etc.
>>
>> Without this, an fsync will reset writeback_index to
>> 0 (its starting index) + however many pages it wrote, which
>> can mess up the progress of periodic flush.
>>
>> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> 
> Have you done any benchmarks with and without this patch series?
> 
> Say, compilebench on a used and mildly fragmented file system?
> 
>      		       	    	       - Ted

Not compilebench specifically, but I did do some benchmarking
with out of cache buffered IO; to be honest I didn't see
striking performance differences, but I did see the writeback
behave better in terms of not wandering all over, even if it
might recover well.

I can try compilebench; do you have specific concerns?

-Eric