You say you can't add RAM or SSDs but that sounds like exactly what you need. If your data doesn't fit in RAM (and, with 25MM records/day, it doesn't), then you

You say you can't add RAM or SSDs but that sounds like exactly what you need. If your data doesn't fit in RAM (and, with 25MM records/day, it doesn't), then you're gonna be hitting those slow HDDs when you run queries. I'd add RAM first and SSDs second but you say you can't do either of those? Well, good luck, I guess -- any "database solution" you choose is going to perform like crap given these resource constraints.