On May 11, 2010, at 5:23 PM, John Gilmore wrote: > There's a classic Unix problem with distribution of disk access times > that relates to how older Unixes did "sync" -- every 30 seconds there was > an instant traffic jam at the interface to the drives. This has been > studied to death; here are some assorted papers: > > http://www.cis.upenn.edu/~jms/usenix.pdf > Has graphs from old DEC hardware that look similar to XO-1.5. > > http://www.usenix.org/events/osdi06/tech/nightingale/nightingale.pdf > Good references section. > > http://www.eecs.umich.edu/Rio/papers/chen96_2.pdf > Studies ext3 performance in depth. > > Most such studies looked at read() times because the usual workload has > far more reads than writes. Odd delays in write() times are interesting.
Writes in UNIX are typically asynchronous --- get the data into a memory buffer and the program can continue along its merry way. I strongly suspect that writes through the SD system are forced to wait for the media to complete the write. wad _______________________________________________ Devel mailing list [email protected] http://lists.laptop.org/listinfo/devel
