Oh by the way there is another cool thing you can do if you do have "proximity awareness": save the bandwidth costs of repair. Suppose you have a petabyte worth of hard drives -- e.g. 500 hard drive each of 2 TB. Hard drives fail at a rate of around 6% per year [1], so you would expect about 30 of them to fail per year, which is about one every two weeks.
Suppose you have three locations which have very fast and cheap bandwidth within the location, e.g. the LAN connecting different servers in the same rack, but slower and more expensive bandwidth between locations, e.g. if you have to pay a per-bandwidth fee to transmit between co-lo's. Then let's call "Q" the number of locations, and you should set your N value to be (K+1) * Q. For example, if K is 3 (three shares needed to reconstruct a file), and Q is 3 (three locations), then set your N equal to 12 and (once we implement "proximity awareness"), make sure that there are four shares in each location. The advantage of this is that it makes repair cheap. Suppose you have a [1] Eduardo Pinheiro, ~Wolf-Dietrich Weber, and Luiz André Barroso (all of Google Inc.): Failure Trends in a Large Disk Drive Population http://labs.google.com/papers/disk_failures.pdf _______________________________________________ tahoe-dev mailing list [email protected] http://allmydata.org/cgi-bin/mailman/listinfo/tahoe-dev
