Supportive of switching the default to mmap_index_only as well. I don’t have numbers handy to share, but my experience has been significantly lower read latency and I wouldn’t run with auto. I’ve also not observed substantial heap pressure after switching - it was strictly an improvement.
- Scott — Mobile > On Sep 6, 2023, at 8:50 AM, Paulo Motta <pauloricard...@gmail.com> wrote: > > > 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
