Hi, I would like to get back to this. I proposed this default configuration change on the user list ~1 month ago and there were no comments [1].
I created CASSANDRA-19021 [2] to make the proposed change and Stefan kindly submitted a patch, CI is looking good. Any objections to making this change in 5.0? If not, we will merge in 24 hours. Thanks, Paulo [1] - https://lists.apache.org/thread/w0gkdj7fhylycqwmd73p0kfck7jr8qth [2] - https://issues.apache.org/jira/browse/CASSANDRA-19021 On Wed, Sep 6, 2023 at 5:12 PM Paulo Motta <pauloricard...@gmail.com> wrote: > > I wonder why disk_access_mode property is not in cassandra.yaml (looking > into trunk right now) > > I think there's a prehistoric reason why it was removed but I can't > remember right now. > > > Do you all think we can add it there with brief explanation what each > option does? > > We could reinclude it as long as we provide a clear recommendation on when > to change from the default since this is an advanced setting which should > be rarely changed. But I still think we should provide a more > stable/foolproof default (mmap_index_only) since the current default (mmap) > is known to cause instability in some scenarios. > > Also there is a technicality with changing the default, if we change the > "auto" behavior from mmap to mmap_index_only this may affect users relying > on the default "mmap" behavior. Not sure the best way to address that, is a > big NEWS note sufficient? Even though users are expected to read NEWS when > upgrading we know well not all users read it. > > > Shall we also share this thread with @user? > > Thanks Ekaterina! If we decide to change the default we can run this > through the user@ list to see what the user community thinks. > > On Wed, Sep 6, 2023 at 4:45 PM Ekaterina Dimitrova <e.dimitr...@gmail.com> > wrote: > >> Thanks for starting this discussion, Paulo! >> >> Shall we also share this thread with @user? >> >> On Wed, 6 Sep 2023 at 16:35, C. Scott Andreas <sc...@paradoxica.net> >> wrote: >> >>> 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 >>> >>>
