Hey, great questions!

Query Performance: Right now, we've got test machines deployed with 8 500GB disks for packets + 1 indexing disk (all 15KRPM spinning disks). They keep at 90% full, or roughly 460GB/disk, about 1K files/disk. Querying over the entire corpus (~4TB of packets) for something innocuous like 'port 65432' takes 25 seconds to return ~50K packets (that's after dropping all disk caches). The same query run again takes 1.5 sec, with disk caches in place. Of course, the number of packets returned is a huge factor in this... each packet requires a seek in the packets file. Searching for something that doesn't exist (host 0.0.0.1) takes roughly 5 seconds. Note that time-based queries, like "port 4444 and after 3h ago and before 1h ago" do choose to only query certain files, taking advantage of the fact that we name files by microsecond timestamp and we flush files every minute.

A big part of query performance is actually over-provisioning disks. We see disk throughput of roughly 160-180MB/s. If we write 160MB/s, our read throughput is awful. If we write 100MB/s, it's pretty good. Who would have thought: disks have limited bandwidth, and it's shared between reads and writes. :)

We actually don't use LevelDB... we use the SSTables that underly LevelDB. Since we know we're write-once, we use https://github.com/google/leveldb/blob/master/include/leveld... directly for writes (and its Go equivalent for reads). I'm familiar with the file format (they're used extensively inside Google), so it was a simple solution. That said, it's been very successful... we tend to have indexes in the 10s of MBs for 2-4GB files. Of course, index size/compressibility is directly correlated with network traffic: more varied IPs/ports would be harder to compress. The built-in compression of LevelDB tables is also a boon here... we get prefix compression on keys, plus snappy compression on packet seek locations, for free.

We currently do no compression of packets. Doing so would definitely increase our CPU usage per packet, and I'm really scared of what it would do to reads. Consider that reading packets in compressed storage would require decompressing each block a packet is in. On the other hand, if someone wanted to store packets REALLY long term, they could easily compress the entire blockfile+index before uploading to more permanent storage. I expect this would be better than having to do it inline. Even if we did build it in, we'd probably do it tiered (initial write uncompressed, then compress later on as possible).

AF_PACKET is no better than PF_RING+DNA, but I also don't think it's any worse. They both have very specific trade-offs. The big draw for me for AF_PACKET is that it's already there... any stock Linux machine will already have it built in and working. Thus steno should "just work", while a PF_RING solution has a slightly higher barrier to entry. I think PF_RING+DNA should give similar performance to steno... but libzero currently probably gives better performance because packets can be shared across processes. This is a really interesting problem that I'm wondering if we could also solve with AF_PACKET... but that's a story for another day. Short story: I wanted this to work on stock linux as much as possible.