I would also like to back this proposal. We change this default because several incidents have occurred by leaving the default of auto. There are rare cases where auto/mmap is the better option but as for a default mmap_index_only is safer.
On Wed, Nov 15, 2023 at 6:35 AM Paulo Motta <pa...@apache.org> wrote: > 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 >>>> >>>>
