OPTIMIZE statement enqueues a RT index for optimization in a background thread.

If OPTION sync=1 is used, the command will wait until the optimization process is done (or if the connection timeout - but the optimization will continue to run).

Over time, RT indexes can grow fragmented into many disk chunks and/or tainted with deleted, but unpurged data, impacting search performance. When that happens, they can be optimized. Basically, the optimization pass merges together disk chunks pairs, purging off documents suppressed by K-list as it goes.

That is a lengthy and IO intensive process, so to limit the impact, all the actual merge work is executed serially in a special background thread, and the OPTIMIZE statement simply adds a job to its queue. Currently, there is no way to check the index or queue status (that might be added in the future to the SHOW INDEX STATUS and SHOW STATUS statements respectively). The optimization thread can be IO-throttled, you can control the maximum number of IOs per second and the maximum IO size with rt_merge_iops and rt_merge_maxiosize directives respectively. The optimization jobs queue is lost on daemon crash.

The RT index being optimized stays online and available for both searching and updates at (almost) all times during the optimization. It gets locked (very) briefly every time that a pair of disk chunks is merged successfully, to rename the old and the new files, and update the index header.

At the moment, OPTIMIZE needs to be issued manually, the indexes will not be optimized automatically. That might change in the future releases.

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