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[Imported from Trac: page TahoeTwo, version 4]

davidsarah 2009-10-19 18:25:09 +00:00
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commit e2feebee38
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TahoeTwo.md

@ -6,12 +6,12 @@ When a file is uploaded, the encoded shares are sent to other peers. But to
which ones? The [PeerSelection](PeerSelection) algorithm is used to make this choice. which ones? The [PeerSelection](PeerSelection) algorithm is used to make this choice.
Early in 2007, we were planning to use the following "Tahoe Two" algorithm. Early in 2007, we were planning to use the following "Tahoe Two" algorithm.
By the time we released 0.2.0, we switched to "tahoe3", but when we released By the time we released 0.2.0, we switched to "TahoeThree", but when we released
v0.6, we switched back (ticket #132). v0.6, we switched back (ticket #132).
As in Tahoe Three, the verifierid is used to consistently-permute the set of As in [TahoeThree](TahoeThree), the verifierid (= [StorageIndex](StorageIndex)) is used to consistently-permute
all peers (by sorting the peers by HASH(verifierid+peerid)). Each file gets a the set of all peers (by sorting the peers by HASH(verifierid+peerid)). Each file
different permutation, which (on average) will evenly distribute shares among gets a different permutation, which (on average) will evenly distribute shares among
the grid and avoid hotspots. the grid and avoid hotspots.
With our basket of (usually 10) shares to distribute in hand, we start at the With our basket of (usually 10) shares to distribute in hand, we start at the
@ -51,8 +51,8 @@ peers we should talk to (perhaps by recording the permuted peerid of the last
node to which we sent a share, or a count of the total number of peers we node to which we sent a share, or a count of the total number of peers we
talked to during upload). talked to during upload).
I suspect that this approach handles churn more efficiently than tahoe3, but I suspect that this approach handles churn more efficiently than [TahoeThree](TahoeThree), but
I haven't gotten my head around the math that could be used to show it. On I haven't gotten my head around the math that could be used to show it. On
the other hand, it takes a lot more round trips to find homes in small meshes the other hand, it takes a lot more round trips to find homes in small meshes
(one per share, whereas tahoe three can do just one per node). (one per share, whereas [TahoeThree](TahoeThree) can do just one per node).