[tahoe-dev] Proximity Aware Decoding

Zooko O'Whielacronx zookog at gmail.com
Wed Dec 9 15:46:57 UTC 2009


ugh, hit the wrong key and sent unfinished mail. holding a baby. --Z

On Wed, Dec 9, 2009 at 8:46 AM, Zooko O'Whielacronx <zookog at gmail.com> wrote:
> 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
>



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