[BitTorrent] A few random questions

shad0wmatt3r mgp at ucla.edu
Fri Jul 30 16:01:52 EDT 2004


First, I've heard quite a few people rave about the disk cache feature
in BitComet. On the web site, it says the default cache size is 8 MB.
Now aside from perusing the web site, I haven't really looked into
BitComet, but does anyone know how this cache is used? Does it cache
pieces that it thinks are likely to be read (by, say, calculating
which pieces it has are the rarest of all [from what it can see within
the swarm] -- and thus most likely to be requested -- and bringing
them into the cache)? Does it simply buffer pieces to be written (so
if we have 256KB pieces, it will put 32 into the buffer before writing
them all to disk at once)? The former doesn't sound too plausible
because some clients don't strictly request the rarest pieces if a
minimum number exist (see Azureus). The latter doesn't sound too
helpful since the pieces to be written all at once are likely
non-contiguous, and so you're still going to have the scattered disk
writes you would have without a cache, although not all at once. I'm
passively working on a p2p system with swarming similar to BitTorrent,
and I'm looking for feedback on whether a cache is a good idea/worth

Second, here are some things I couuldn't find in the BitTorrent
protocol specification. These questions are more to satisfy my own
curiosity than anything...
a. Can a peer request multiple pieces from another peer at a time, or
can it only request a new piece once the current piece has finished
b. If a peer can request multiple pieces at a time, is there proper
etiquette (or some limit) for the number of outstanding pieces you can
request, or have queued for sending at the other peer?
c. Can a peer request multiple piece blocks (err, intra-piece pieces)
from another peer at a time, or not (similar question to a)?
d. Do clients hard-code the size of a piece block, or scale it
proportionally with the piece size?

Thanks for the help everyone =)

- Michael Parker

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