Add comment explaining coro threads

fdbserver/KeyValueStoreRocksDB.actor.cpp

@@ -370,6 +370,16 @@ struct RocksDBKeyValueStore : IKeyValueStore {
 	std::unique_ptr<rocksdb::WriteBatch> writeBatch;
 
 	explicit RocksDBKeyValueStore(const std::string& path, UID id) : path(path), id(id) {
+		// In simluation, run the reader/writer threads as Coro threads (i.e. in the network thread. The storage engine
+		// is still multi-threaded as background compaction threads are still present. Reads/writes to disk will also
+		// block the network thread in a way that would be unacceptable in production but is a necessary evil here. When
+		// performing the reads in background threads in simulation, the event loop thinks there is no work to do and
+		// advances time faster than 1 sec/sec. By the time the blocking read actually finishes, simulation has advanced
+		// time by more than 5 seconds, so every read fails with a transaction_too_old error. Doing blocking IO on the
+		// main thread solves this issue. There are almost certainly better fixes, but my goal was to get a less
+		// invasive change merged first and work on a more realistic version if/when we think that would provide
+		// substantially more confidence in the correctness.
+		// TODO: Adapt the simulation framework to not advance time quickly when background reads/writes are occurring.
 		if (g_network->isSimulated()) {
 			writeThread = CoroThreadPool::createThreadPool();
 			readThreads = CoroThreadPool::createThreadPool();