There are lots of well-written articles, such as this [westnet.com] and especially this [kerneltrap] on Page Cacheing and pdflush, but RackerHacker (although the title says “reads”, it really seems to address lots of small writes) summarises it very well:
vm.dirty_ratio – The highest % of your memory that can be used to hold dirty data. If you set this to a low value, the kernel will flush small writes to the disk more often. Higher values allow the small writes to stack up in memory. They’ll go to the disk in bigger chunks.
vm.dirty_background_ratio – The lowest % of your memory where pdflush is told to stop when it is writing dirty data. You’ll want to keep this set as low as possible.