On Sat, Feb 6, 2010 at 8:44 AM, Mark Kettenis <mark.kette...@xs4all.nl> wrote: >> > Continuing in my "make IO suck less" phase, cp would be a lot more >> > efficient if it didn't bounce the disk heads around so much. Instead of >> > using a tiny 64k buffer, use an amount based on a small fraction of RAM. >> >> Isn't it the task of the buffer cache to optimize memory use here? > > Exactly the point I was just about to raise.
Optimizing memory has nothing to do with it. The buffer cache doesn't do a whole lot to reduce seeking. This is what cp looks like: read .000001s seek .001s write .000001s seek .001s read .000001s seek .001s It doesn't matter if the buffer cache does readahead, it never reads enough ahead. It doesn't matter if the buffer cache is write back, the writes are still in the queue, and the elevator still goes. If you were moving from the 10th floor to the 90th floor, would you take the boxes up one at a time?
