During the ingestion (writes)
its possible that the writes may slow down the reads in that DC1
Let us take an alternative XDR configurtion
DC1 and DC2 and DC3
We have only reads from DC1 and DC2
and writes only from DC3
Replication is turned ON only for a short duration
after data in written / ingested into DC3
in a small time window
Would the alternative XDR configuration help in avoiding slower reads during writes?
I can compare this alternative configuration which only generates fast to merge and replicate deltas from DC3
The XDR experts suggest it is not a good idea to turn replication on only for a short duration as that would lead to stale reads at other DCs. Moreover if there are a large volume of writes and the window to replicate is small, the system may fall back to recovery mode which is inefficient and should be avoided.
The suggestion is to confirm if the read performance is a real problem by running your workload in the 2-DC configuration.
If read performance is an issue, you may want to consider increasing the capacity of the two DCs and see if that solves the problem.
A third dedicated DC for writes may have some advantages at the cost of additional complexity of configuration and management. It may collapse some writes (depending on your workload if same keys are updated multiple times) before replicating and therefore could result in fewer overall writes to the “read” clusters.
In summary, since your workload and acceptable read performance would govern the best solution, test the potential configurations in the order of their complexity and cost.