Binary logs are essentially a recovery mechanism. With binary logs enabled, searchd writes every given transaction to the binlog file, and uses that for recovery after an unclean shutdown. On clean shutdown, RAM chunks are saved to disk, and then all the binlog files are unlinked.

During normal operation, a new binlog file will be opened every time when binlog_max_log_size limit (which defaults to 128M) is reached. Older, already closed binlog files are kept until all of the transactions stored in them (from all indexes) are flushed as a disk chunk. Setting the limit to 0 pretty much prevents binlog from being unlinked at all while searchd is running; however, it will still be unlinked on clean shutdown.

There are 3 different binlog flushing strategies, controlled by binlog_flush directive which takes the values of 0, 1, or 2. 0 means to flush the log to OS and sync it to disk every second; 1 means flush and sync every transaction; and 2 (the default mode) means flush every transaction but sync every second. Sync is relatively slow because it has to perform physical disk writes, so mode 1 is the safest (every committed transaction is guaranteed to be written on disk) but the slowest. Flushing log to OS prevents from data loss on searchd crashes but not system crashes. Mode 2 is the default.

On recovery after an unclean shutdown, binlogs are replayed and all logged transactions since the last good on-disk state are restored. Transactions are checksummed so in case of binlog file corruption garbage data will not be replayed; such a broken transaction will be detected and, currently, will stop replay. Transactions also start with a magic marker and timestamped, so in case of binlog damage in the middle of the file, it's technically possible to skip broken transactions and keep replaying from the next good one, and/or it's possible to replay transactions until a given timestamp (point-in-time recovery), but none of that is implemented yet as of 1.10-beta.

One unwanted side effect of binlogs is that activel updating a small RT index that fully fits into a RAM chunk part will lead to an ever-growing binlog that can never be unlinked until clean shutdown. Binlogs are essentially append-only deltas against the last known good saved state on disk, and unless RAM chunk gets saved, they can not be unlinked. An ever-growing binlog is not very good for disk use and crash recovery time. Starting with 2.0.1-beta you can configure searchd to perform a periodic RAM chunk flush to fix that problem using a rt_flush_period directive. With periodic flushes enabled, searchd will keep a separate thread, checking whether RT indexes RAM chunks need to be written back to disk. Once that happens, the respective binlogs can be (and are) safely unlinked.

Note that rt_flush_period only controls the frequency at which the checks happen. There are no guarantees that the particular RAM chunk will get saved. For instance, it does not make sense to regularly re-save a huge RAM chunk that only gets a few rows worh of updates. The search daemon determine whether to actually perform the flush with a few heuristics.