I'm in a RHEL8, Splunk 8.2.6 distributed environment with single kvstore on each servers. Can Memory Mapped (MMAP) storage engine and WiredTiger storage engine exist in the same Splunk environment but on different servers in single instance? I will like to do the migration but one server at a time over multiple days.
Can you be more precise what at your splunk environment looks like? If you mean you are running multiple single search head instances there is no communication between the kvstores and migrating vom mmap to wiredtiger one-by-one shouldn't be any issue.
if you are kvstore heavily or big dataset try performance in a test environment. We have seen increases in search time up to 3x.