When TREX updates an index, it rewrites the majority of the index files. If the indexes are large this process can take a long time and generate a high system load.
TREX allows you to activate a delta index in order to speed up the update. The delta index is a separate index that TREX generates in addition to the main index. The main index and its delta index only differ TREX-internally. Outside of TREX they form a unit.
If the delta index is activated, changes flow into the delta index. Because the delta index is smaller than the main index, fewer files are affected by updates. The delta index can therefore be updated more quickly.
The delta index is deactivated by default. The following rules are valid for its activation:
If you have a single host system the activation is optional. However, it is recommended if the main index has reached a certain size. If you activate the delta index to soon, performance does not improve.
If you have a distributed TREX system the activation is obligatory. However, you still only activate it once the main index has reached a certain size.
Activating the delta index doesn't only speed up the update of the master index - it also enables fast index replication with a low network load.
When index replication takes place the master index server replicates all changed master index files. Because the delta index consists of fewer files, it naturally has fewer files to replicate. This means that index replication is quicker. Moreover, if you have decentralized data storage the network load is also less because TREX has to copy less files to the slave hosts.
The delta index only speeds up the update if it is kept small. If it becomes too large, it no longer improves performance. When it reaches a certain size you have to integrate it in the main index. You can integrate the delta index manually or configure TREX so that TREX regularly integrates it automatically. TREX creates a new delta index automatically when the integration of the previous delta index is complete.