FLUSH TABLE rt_tableFLUSH TABLE forcefully flushes RT table RAM chunk contents to disk.
The real-time table RAM chunk is automatically flushed to disk during a clean shutdown, or periodically every rt_flush_period seconds.
Issuing a FLUSH TABLE command not only forces the RAM chunk contents to be written to disk but also triggers the cleanup of binary log files.
- SQL
FLUSH TABLE rt;Query OK, 0 rows affected (0.05 sec)Over time, RT tables may become fragmented into numerous disk chunks and/or contaminated with deleted, yet unpurged data, affecting search performance. In these cases, optimization is necessary. Essentially, the optimization process combines pairs of disk chunks, removing documents that were previously deleted using DELETE statements.
Beginning with Manticore 4, this process occurs automatically by default. However, you can also use the following commands to manually initiate table compaction.
OPTIMIZE TABLE table_name [OPTION opt_name = opt_value [,...]]OPTIMIZE statement adds an RT table to the optimization queue, which will be processed in a background thread.
- SQL
OPTIMIZE TABLE rt;By default, OPTIMIZE merges the RT table's disk chunks down to a number equal to # of CPU cores * 2. You can control the number of optimized disk chunks using the cutoff option.
Additional options include:
- Server setting optimize_cutoff for overriding the default threshold
- Per-table setting optimize_cutoff
- SQL
OPTIMIZE TABLE rt OPTION cutoff=4;When using OPTION sync=1 (0 by default), the command will wait for the optimization process to complete before returning. If the connection is interrupted, the optimization will continue running on the server.
- SQL
OPTIMIZE TABLE rt OPTION sync=1;Optimization can be a lengthy and I/O-intensive process. To minimize the impact, all actual merge work is executed serially in a special background thread, and the OPTIMIZE statement simply adds a job to its queue. The optimization thread can be I/O-throttled, and you can control the maximum number of I/Os per second and the maximum I/O size with the rt_merge_iops and rt_merge_maxiosize directives, respectively.
During optimization, the RT table being optimized remains online and available for both searching and updates nearly all the time. It is locked for a very brief period when a pair of disk chunks is successfully merged, allowing for the renaming of old and new files and updating the table header.
As long as auto_optimize is not disabled, tables are optimized automatically.
If you are experiencing unexpected SSTs or want tables across all nodes of the cluster to be binary identical, you need to:
- Disable auto_optimize.
- Manually optimize tables:
On one of the nodes, drop the table from the cluster:
- SQL
SQL📋ALTER CLUSTER mycluster DROP myindex;Optimize the table:
- SQL
SQL📋OPTIMIZE TABLE myindex;Add back the table to the cluster:
- SQL
SQL📋ALTER CLUSTER mycluster ADD myindex;When the table is added back, the new files created by the optimization process will be replicated to the other nodes in the cluster. Any local changes made to the table on other nodes will be lost.
Table data modifications (inserts, replaces, deletes, updates) should either:
- Be postponed, or
- Be directed to the node where the optimization process is running.
Note that while the table is out of the cluster, insert/replace/delete/update commands should refer to it without the cluster name prefix (for SQL statements or the cluster property in case of an HTTP JSON request), otherwise they will fail. Once the table is added back to the cluster, you must resume write operations on the table and include the cluster name prefix again, or they will fail.
Search operations are available as usual during the process on any of the nodes.
Manticore provides isolation during the flushing and merging process of a real-time table to prevent any changes from affecting running queries.
For example, during table compaction, a pair of disk chunks are merged and a new chunk is produced. At one point, a new version of the table is created with the new chunk replacing the original pair. This is done seamlessly so that a long-running query using the original chunks will continue to see the old version of the table, while a new query will see the new version with the resulting merged chunk.
The same applies to flushing a RAM chunk, where suitable RAM segments are merged into a new disk chunk and the participated RAM chunk segments are abandoned. During this operation, Manticore provides isolation for queries that started before the operation began.
Furthermore, these operations are transparent for replaces and updates. If you update an attribute in a document that belongs to a disk chunk being merged with another one, the update will be applied to both that chunk and the resulting merged chunk. If you delete a document during a merge, it will be deleted in the original chunk and also in the resulting merged chunk, which will either have the document marked as deleted or have no such document at all if the deletion happened early in the merging process.