Memory modules of different sizes can be mixed provided that other memory population rules are followed (for example, 2 GB and 4 GB memory modules can be mixed).
UDIMMs, RDIMMs, and LRDIMMs must not be mixed
(page 46 more General Memory Module Installation Guidelines)
DELL-Charles R
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April 7th, 2022 13:00
Hello mikeprag,
The R820 does support Flexible Memory Configuration.
You should be able to put the largest DIMMs in the 1 and 2 slot for each processor .
I'd recommend looing over the Memory guidelines to see you match up.
Owner manual : https://dell.to/3x8EKiH
System Memory Page 43-51
Memory modules of different sizes can be mixed provided that other memory population rules are followed (for example, 2 GB and 4 GB memory modules can be mixed).
UDIMMs, RDIMMs, and LRDIMMs must not be mixed
(page 46 more General Memory Module Installation Guidelines)