[tahoe-dev] Erasure coding in Tahoe

[Russ Weeks]

Hi, all.

I have a couple questions regarding Reed-Solomon codes and their use
in Tahoe.  I see in "Tahoe - The Least-Authority Filesystem" that
erasure coding is based on 2 parameters: N, the total number of shares
that will be written, and K, the number of shares to be used on read.
Does this imply that the data being written can sustain N-K failures?

I know that the rateless erasure code folks use the term "systematic"
to mean an erasure code where all the input symbols appear in the
output symbols.  I don't know if that term is widely used, but
Reed-Solomon is a systematic erasure code, isn't it?  I mean, in the
absence of failure, I'll always choose to read from my data strips
instead of my parity or check strips, right?

In the presence of a failure, when I need to rebuild one or more
strips, is it true that I always need to read K strips to rebuild
those strips?  I mean, please help me consider the following scenario:

1.  I have 10 storage nodes in my system; 10TB of data (prior to
erasure coding) is spread evenly across all 10 nodes and I can suffer
1 failure
2.  Some lunatic attacks one of my storage nodes with an axe. :)  I
commission a new, empty node to take its place.
3.  Is it true that, in aggregate, I need to move 10TB of data around
my distributed system to produce the 1TB of data for the new node?

Thanks very much for any help,

Russ