Flushing RAM chunk to a new disk chunk



FLUSH RAMCHUNK forcibly creates a new disk chunk in an RT index.

Normally, RT index would flush and convert the contents of the RAM chunk into a new disk chunk automatically, once the RAM chunk reaches the maximum allowed rt_mem_limit size. However, for debugging and testing it might be useful to forcibly create a new disk chunk, and FLUSH RAMCHUNK statement does exactly that.

Note that using FLUSH RAMCHUNK increases RT index fragmentation. Most likely, you want to use FLUSH RTINDEX instead. We suggest that you abstain from using just this statement unless you're absolutely sure what you're doing. As the right way is to issue FLUSH RAMCHUNK with following OPTIMIZE command. Such combo allows to keep RT index fragmentation on minimum.

  • SQL
Query OK, 0 rows affected (0.05 sec)

Flushing RAM chunk to disk



FLUSH RTINDEX forcibly flushes RT index RAM chunk contents to disk.

Backing up an RT index is as simple as copying over its data files, followed by the binary log. However, recovering from that backup means that all the transactions in the log since the last successful RAM chunk write would need to be replayed. Those writes normally happen either on a clean shutdown, or periodically with a (big enough!) interval between writes specified in rt_flush_period directive. So such a backup made at an arbitrary point in time just might end up with way too much binary log data to replay.

FLUSH RTINDEX forcibly writes the RAM chunk contents to disk, and also causes the subsequent cleanup of (now redundant) binary log files. Thus, recovering from a backup made just after FLUSH RTINDEX should be almost instant.

  • SQL
Query OK, 0 rows affected (0.05 sec)

Compacting an index

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.


OPTIMIZE INDEX index_name [OPTION opt_name = opt_value [,...]]

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

Number of optimized disk chunks

The optimize process reduces the disk chunks by default to a number equal with # of CPU cores * 2. The number of optimized disk chunks can be controlled with option cutoff.

In previous releases OPTIMIZE reduced the disk chunks to a single one. This can still be achieved if desired by setting OPTION cutoff=1.

Running in foreground

If OPTION sync=1 is used (0 by default), the command will wait until the optimization process is done (in case the connection interrupts the optimization will continue to run on the server).

Throttling the IO impact

Optimize can be 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 RT index being optimized stays online and available for both searching and updates at (almost) all times during the optimization. It gets locked for a very short time when 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 are not optimized automatically. It will be changed in future releases.

  • SQL
Query OK, 0 rows affected (0.00 sec)

Optimizing clustered indexes

Currently indexes cannot be optimized directly while are being part of a cluster.

The following procedure should be used:

On one of the nodes drop the index from the cluster:

mysql> ALTER CLUSTER mycluster DROP myindex;

Optimize the index:

mysql> OPTIMIZE INDEX myindex;

Add back the index to the cluster:

mysql> ALTER CLUSTER mycluster ADD myindex;

When the index is added back, the new files created by the optimize process will be replicated to the other nodes in the cluster. Any changes made locally to the index on other nodes will be lost.

Writes on the index should either be stopped or directed to the node were the optimize process is running. Note that after the index is out of the cluster, writes must be made locally and the index name must not contain the cluster name as prefix (for SQL statements or cluster property for HTTP requests). As soon as the index is added back to the cluster, writes can be resumed. At this point the writes operations on the index must include (again) the cluster prefix (for SQL statements or cluster property for HTTP requests). Searches will be available as usual during the process on any of the nodes.

In future releases it's expected to remove the need of this process and simply perform OPTIMIZE without the need to take the index out of the cluster.