Swap sticks in pairs, not one by one.

Do it in pairs. The DL380 Gen9 uses quad-channel memory per CPU, and mixing unmatched sticks (different size, rank, or timing) often forces the system to run in a lower-performance mode.

You don’t have to do all at once—just don’t swap a single DIMM. Pull the two sticks from the same channel pair (e.g., slots A1 and A2 for CPU1) and replace them with matched ones. Repeat for each pair until you hit 128GB. The server will reinitialize after each boot and automatically adjust.

If you actually want full speed, stick to 2R x4 16GB or 32GB DIMMs, all same spec. Mixing ranks or sizes across pairs is where you lose performance.

The Gen9 iLO will complain about “memory configuration not optimal” if you do it wrong. Listen to it.