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57 lines
3.4 KiB
Plaintext
= THIS PAGE DESCRIBES HISTORICAL DESIGN CHOICES. SEE docs/architecture.rst FOR CURRENT DOCUMENTATION =
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When a file is uploaded, the encoded shares are sent to other peers. But to
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which ones? The PeerSelection algorithm is used to make this choice.
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Early in 2007, we were planning to use the following "Tahoe Two" algorithm.
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By the time we released 0.2.0, we switched to "tahoe3", but when we released
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v0.6, we switched back (ticket #132).
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As in Tahoe Three, the verifierid is used to consistently-permute the set of
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all peers (by sorting the peers by HASH(verifierid+peerid)). Each file gets a
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different permutation, which (on average) will evenly distribute shares among
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the grid and avoid hotspots.
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With our basket of (usually 10) shares to distribute in hand, we start at the
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beginning of the list and ask each peer in turn if they are willing to hold
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on to one of our shares (the "lease request"). If they say yes, we remove
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that share from the basket and remember who agreed to host it. Then we go to
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the next peer in the list and ask them the same question about another share.
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If a peer says no, we remove them from the list. If a peer says that they
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already have one or more shares for this file, we remove those shares from
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the basket. If we reach the end of the list, we start again at the beginning.
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If we run out of peers before we run out of shares, we fail unless we've
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managed to place at least some number of the shares: the likely threshold is
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to attempt to place 10 shares (out of which we'll need 3 to recover the
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file), and be content if we can find homes for at least 7 of them.
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In small networks, this approach will loop around several times and place
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several shares with each node (e.g. in a 5-host network with plenty of space,
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each node will get 2 shares). In large networks with plenty of space, the
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shares will be placed with the first 10 peers in the permuted list. In large
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networks that are somewhat full, we'll need to traverse more of the list
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before we find homes for the shares. The average number of peers that we'll
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need to talk to is vaguely equal to 10 / (1-utilization), with a bunch of
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other terms that relate to the distribution of free space on the peers and
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the size of the shares being offered. Small files with small shares will fit
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anywhere, large files with large shares will only fit on certain peers, so
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the mesh may have free space but no holes large enough for a very large file,
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which might indicate that we should try again with a larger number of
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(smaller) shares.
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When it comes time to download, we compute a similar list of permuted
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peerids, and start asking for shares beginning with the start of the list.
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Each peer gives us a list of the shareids that they are holding. Eventually
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(depending upon how much churn the peerlist has experienced), we'll find
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holders for at least 3 shares, or we'll run out of peers. If the mesh is very
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large and we want to fail faster, we can establish an upper bound on how many
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peers we should talk to (perhaps by recording the permuted peerid of the last
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node to which we sent a share, or a count of the total number of peers we
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talked to during upload).
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I suspect that this approach handles churn more efficiently than tahoe3, but
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I haven't gotten my head around the math that could be used to show it. On
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the other hand, it takes a lot more round trips to find homes in small meshes
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(one per share, whereas tahoe three can do just one per node).
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