Basic Problem
Most file systems now have large memory caches (buffers) to hold recently-accessed blocks
Most reads are thus satisfied from the buffer cache
From the point of view of the disk, most traffic is write traffic
So to speed up disk I/O, we need to make writes go faster
But disk performance is limited ultimately by disk head movement
With current file systems, adding a block takes several writes (to the file and to the metadata), requiring several disk seeks