HBase might be a good solution, too, but I don't know a whole lot about it. CouchDB would work fine but you'd want to add some operational automation and a prox

HBase might be a good solution, too, but I don't know a whole lot about it. CouchDB would work fine but you'd want to add some operational automation and a proxy to roll over between temporary databases since deleting old logs from a single database won't fully reduce its size and, as stated above, compaction isn't terribly fun or fast but you can avoid it by just flat out deleting old log databases.