Splunk does not use arealtional database to store events and indexes.
The storage is all flat file based.
Please have a look here:
Hope that answers your question?
is it mean, Splunk develop its own system to do this from ZERO? And it is really does not have any kind direct/significant relation to other DB technology?
Yes, Splunk developed their own on-disk storage format from "zero". (If you call having a C++ compiler and standard libraries "zero") From an architecture perspective, there are large differences between an ACID-capable generalized RDBMS and (essentially) a search engine's data storage. Splunk does not have (and does not need) many of the features a relational DB has. Also, most relational DB's full-text search are ugly side-additions. The Splunk developers were able to make an on-disk data format that meets their needs exactly.
Splunk might be using mongDB database, not sure even i want confirmation.
Why mongoDB: Coz i have seen the process named mongoDB running when indexer starts or restart.
Also source = C:\Program Files\Splunk\var\log\splunk\mongod.log with index=_internal*