Database server: Small quick RAM or large slow RAM?
You will want to go with the large and slow RAM. The difference in RAM performance is negligible compared to the difference between RAM performance and disk performance.
Alright, it's very very very simple :
Does your database fit in 48GB of RAM with OS and all ? if yes, take that. Else, take 96GB
Also, database fitting in xyz GB of RAM means it fits with index's, views and all that.
SSD comments are complete utter nonsense, both the bandwidth and access time are not on the same level and no SSD can justify taking less RAM.
Database only? Depending on the database, I would think the larger RAM would be better. Speed difference has been proven to be miniscule at best, but the additional 48gb will / may make a huge difference.
Definitely large RAM, speed be damned.
Access to random data for RAM technology from XX century '90 is below 100 ns. That's using practically ancient chips that won't even physically fit into anything borderline contemporary.
Access to random data for cutting edge 15k rpm hard drives is in measured in miliseconds. 100 ns is 10 000 times shorter (nano -> micro -> milli) than 1 ms. Current RAM is faster, and HDD needs several milliseconds to access data. I couldn't care less if my RAM was 50 000 faster or only 30 000 times faster than HDD, if I could get more.
You must take your attention in some points:
- Memory lantecy Memory speed depends on two factors: bus speed and latency. Usally chips with more density result in a higher latency, wich finaly means less speed
- Total index data The most critical y to load the entire index data into memory. Index data is the most critical data you need in memory (higher penalty effect in performance).
- Disk speed Do you have the DB data stored in SSD? If the answer is yes, take care specially of memory latency.