I've no idea what your constraints are, but splitting the index into more manageable chunks, writing to multiple masters and reading from n slaves off each is an approach that has worked quite well for me (40 million plus records, big lumps of user generated content, total index about 100 GB iirc).

You sacrifice on the accuracy of IDF+, but gain some robustness as result too.

If the BitTorrent approach doesn't work too well, you might consider something similar. I've jotted down a few reading resources for scaling Solr++ but I should probably do a write up of the architecture I built for Boards.ie.

[1] http://wiki.apache.org/solr/DistributedSearch

[2] http://rossduggan.ie/blog/technology/reading-list-for-scalin...