Hi, I've been bitten by OOMs with disk_access_mode:auto/mmap that were fixed by changing to disk_access_mode:mmap_index_only. In a particular benchmark I got 5x more read throughput on 3.11.x with disk_access_mode: mmap_index_only vs disk_access_mode: auto/mmap.
Changing disk_access_mode to mmap_index_only seems to be a common recommendation on forums[1][2][3][4] and slack (find by searching disk_access_mode in the #cassandra channel on https://the-asf.slack.com/). It's not clear to me when using the default disk_access_mode:auto/mmap is beneficial, perhaps only when the read set fits in memory? Mick seems to think on CASSANDRA-15531 [5], that mmap_index_only has a higher heap cost and should be only used when warranted. However it's not uncommon to see people being bitten with OOMs or lower read performance due to the default disk_access_mode, so it makes me think it's not the best fool-proof default. Should we consider changing default "auto" behavior of "disk_access_mode" to be "mmap_index_only" instead of "mmap" in 5.0 since it's likely safer and perhaps more performant? Thanks, Paulo [1] https://stackoverflow.com/questions/72272035/troubleshooting-and-fixing-cassandra-oom-issue [2] https://phabricator.wikimedia.org/T137419 [3] https://stackoverflow.com/a/55975471 [4] https://support.datastax.com/s/article/FAQ-Use-of-disk-access-mode-in-DSE-51-and-earlier [5] https://issues.apache.org/jira/browse/CASSANDRA-15531
