On Wed, Dec 14, 2011 at 10:00:25PM +0800, Wu Fengguang wrote:

quoted

The worst case happens for the USB key, where both old/new kernels
see ~10% worse performance for data=writeback.

quoted

                    ext4                   ext4:wb
------------------------  ------------------------
                    6.20       -10.6%         5.54  fat/UKEY-thresh=100M/ext4-1dd-1-3.2.0-rc3-pause6+

Some more comparison numbers for the above worst case.

I don't see obvious differences from the balance_dirty_pages graphs,

Ah there seem to be many more blocks in write_begin(), indicated by
the more negative pause times in the attached second graph.

Thanks,
Fengguang