Should I go for dtSearch or IDOL for an M-Files self-hosted environment (<2million text files, ~15TB)?

dtSearch is generally recommended for vaults with up to roughly one million documents: Recommendation for Maximum Number of Objects in a Vault When Using DtSearch as a Search Engine

If Smart Search is included in the subscription, I would primarily recommend using that instead. Smart Search does not require any additional server infrastructure to be purchased or maintained, which typically makes it more cost‑effective and significantly simplifies the overall architecture compared to IDOL.

If you need further guidance, the Partner Support team will be happy to help.

Yes, generally Smart Search would be the ideal engine, but the solution would be on-premises and air-gapped environment. If we are to go with IDOL, is it necessary to have another server as detailed in the hardware requirements page?

Alright, that changes things a bit.

One option would be to start with dtSearch and evaluate how it performs. There are many factors that influence this, such as the specific use case (for example, archive vaults typically don't require the same level of performance) and users' subjective performance expectations.

IDOL requires its own dedicated server infrastructure if you decide to go down that route.