I understand and agree. It is just that it would be cool if we avoided the situation when there is a figurative ABC company which has these "bash scripts removing snapshots from cron by rm -rf every second Sunday at 3:00 am" because "that was their workflow for ages".
I am particularly sensitive to this as Cassandra is very cautious when it comes to not disrupting the workflows already out there. I do not know how frequent this would be and if somebody started to complain. I mean ... they could still remove it by hand, right? It is just listsnapshots would not be relevant anymore without refreshing it. I think that might be acceptable. It would be something else if we flat out made manual deletion forbidden, which it is not. On Fri, Aug 9, 2024 at 4:50 PM Bowen Song via dev <dev@cassandra.apache.org> wrote: > If we have the documentation in place, we can then consider the cache to > be the master copy of metadata, and rely on it to be always accurate and up > to date. If someone deletes the snapshot files from filesystem, they can't > complain about Cassandra stopped working correctly - which is the same if > they had manually deleted some SSTable files (they shouldn't). > On 09/08/2024 11:16, Štefan Miklošovič wrote: > > We could indeed do that. Does your suggestion mean that there should not > be a problem with caching it all once explicitly stated like that? > > On Fri, Aug 9, 2024 at 12:01 PM Bowen Song via dev < > dev@cassandra.apache.org> wrote: > >> Has anyone considered simply updating the documentation saying this? >> >> "Removing the snapshot files directly from the filesystem may break >> things. Always use the `nodetool` command or JMX to remove snapshots." >> On 09/08/2024 09:18, Štefan Miklošovič wrote: >> >> If we consider caching it all to be too much, we might probably make >> caching an option an admin would need to opt-in into? There might be a flag >> in cassandra.yaml, once enabled, it would be in memory, otherwise it would >> just load it as it was so people can decide if caching is enough for them >> or they want to have it as it was before (would be by default set to as it >> was). This puts additional complexity into SnapshotManager but it should be >> in general doable. >> >> Let me know what you think, I would really like to have this resolved, >> 18111 brings a lot of code cleanup and simplifies stuff a lot. >> >> On Wed, Aug 7, 2024 at 11:30 PM Josh McKenzie <jmcken...@apache.org> >> wrote: >> >>> If you have a lot of snapshots and have for example a metric monitoring >>> them and their sizes, if you don’t cache it, creating the metric can cause >>> performance degradation. We added the cache because we saw this happen to >>> databases more than once. >>> >>> I mean, I believe you, I'm just surprised querying out metadata for >>> files and basic computation is leading to hundreds of ms pause times even >>> on systems with a lot of files. Aren't most / all of these values cached at >>> the filesystem layer so we're basically just tomato / tomahto caching >>> systems, either one we maintain or one the OS maintains? >>> >>> Or is there really just a count of files well outside what I'm thinking >>> here? >>> >>> Anyway, not trying to cause a ruckus and make needless noise, trying to >>> learn. ;) >>> >>> >>> On Wed, Aug 7, 2024, at 3:20 PM, Štefan Miklošovič wrote: >>> >>> >>> >>> On Wed, Aug 7, 2024 at 6:39 PM Yifan Cai <yc25c...@gmail.com> wrote: >>> >>> With WatcherService, when events are missed (which is to be expected), >>> you will still need to list the files. It seems to me that WatcherService >>> doesn't offer significant benefits in this case. >>> >>> >>> Yeah I think we leave it out eventually. >>> >>> >>> Regarding listing directory with a refresh flag, my concern is the >>> potential for abuse. End-users might/could always refresh before listing, >>> which could undermine the purpose of caching. Perhaps Jeremiah can provide >>> more insight on this. >>> >>> >>> Well, by default, it would not be refreshed every single time. You would >>> need to opt-in into that. If there is a shop which has users with a direct >>> access to the disk of Cassandra nodes and they are removing data manually, >>> I do not know what to say, what is nodetool clearsnapshot and jmx methods >>> good for then? I do not think we can prevent people from shooting into >>> their feet if they are absolutely willing to do that. >>> >>> If they want to refresh that every time, that would be equal to the >>> current behavior. It would be at most as "bad" as it is now. >>> >>> >>> IMO, caching is best handled internally. I have a few UX-related >>> questions: >>> - Is it valid or acceptable to return stale data? If so, end-users have >>> to do some form of validation before consuming each snapshot to account for >>> potential deletions. >>> >>> >>> answer below >>> >>> - Even if listsnapshot returns the most recent data, is it possible that >>> some of the directories get deleted when end-users are accessing them? I >>> think it is true. It, then, enforces end-users to do some validation first, >>> similar to handling stale data. >>> >>> >>> I think that what you were trying to say is that when at time T0 >>> somebody lists snapshots and at T1 somebody removes a snapshot manually >>> then the list of snapshots is not actual anymore? Sure. That is a thing. >>> This is how it currently works. >>> >>> Now, we want to cache them, so if you clear a snapshot which is not >>> physically there because somebody removed it manually, that should be a >>> no-op, it will be just removed from the internal tracker. So, if it is at >>> disk and in cache and you clear it, then all is fine. It is fine too if it >>> is not on disk anymore and you clear it, then it is just removed >>> internally. It would fail only in case you want to remove a snapshot which >>> is not cached, regardless whether it is on disk or not. The deletion of >>> non-existing snapshot ends up with a failure, nothing should be changed in >>> that regard, this is the current behavior too. >>> >>> I want to say that I did not write it completely correctly at the very >>> beginning of this thread. Currently, we are caching only _expiring_ >>> snapshots, because we need to know what is their time of removal so we act >>> on it later. _normal_ snapshots are _not_ cached _yet_. I spent so much >>> time with 18111 that I live in a reality where it is already in, I forgot >>> this is not actually in place yet, we are very close to that. >>> >>> OK thank you all for your insights, I will NOT use inotify. >>> >>> >>> Just my 2 cents. >>> >>> - Yifan >>> >>> On Wed, Aug 7, 2024 at 6:03 AM Štefan Miklošovič <smikloso...@apache.org> >>> wrote: >>> >>> Yes, for example as reported here >>> >>> https://issues.apache.org/jira/browse/CASSANDRA-13338 >>> >>> People who are charting this in monitoring dashboards might also hit >>> this. >>> >>> On Wed, Aug 7, 2024 at 2:59 PM J. D. Jordan <jeremiah.jor...@gmail.com> >>> wrote: >>> >>> If you have a lot of snapshots and have for example a metric monitoring >>> them and their sizes, if you don’t cache it, creating the metric can cause >>> performance degradation. We added the cache because we saw this happen to >>> databases more than once. >>> >>> > On Aug 7, 2024, at 7:54 AM, Josh McKenzie <jmcken...@apache.org> >>> wrote: >>> > >>> > >>> >> >>> >> Snapshot metadata are currently stored in memory / they are cached so >>> we do not need to go to disk every single time we want to list them, the >>> more snapshots we have, the worse it is. >>> > Are we enumerating our snapshots somewhere on the hot path, or is this >>> performance concern misplaced? >>> > >>> >> On Wed, Aug 7, 2024, at 7:44 AM, Štefan Miklošovič wrote: >>> >> Snapshot metadata are currently stored in memory / they are cached so >>> we do not need to go to disk every single time we want to list them, the >>> more snapshots we have, the worse it is. >>> >> >>> >> When a snapshot is _manually_ removed from disk, not from nodetool >>> clearsnapshot, just by rm -rf on a respective snapshot directory, then such >>> snapshot will be still visible in nodetool listsnapshots. Manual removal of >>> a snapshot might be done e.g. by accident or by some "impatient" operator >>> who just goes to disk and removes it there instead of using nodetool or >>> respective JMX method. >>> >> >>> >> To improve UX here, what I came up with is that we might use Java's >>> WatchService where each snapshot dir would be registered. WatchService is >>> part of Java, it uses inotify subsystem which is what Linux kernel offers. >>> The result of doing it is that once a snapshot dir is registered to be >>> watched and when it is removed then we are notified about that via inotify >>> / WatchService so we can react on it and remove the in-memory >>> representation of that so it will not be visible in the output anymore. >>> >> >>> >> While this works, there are some questions / concerns >>> >> >>> >> 1) What do people think about inotify in general? I tested this on >>> 10k snapshots and it seems to work just fine, nevertheless there is in >>> general no strong guarantee that every single event will come through, >>> there is also a family of kernel parameters around this where more tuning >>> can be done etc. It is also questionable how this will behave on other >>> systems from Linux (Mac etc). While JRE running on different platforms also >>> implements this, I am not completely sure these implementations are >>> quality-wise the same as for Linux etc. There is a history of >>> not-so-quality implementations for other systems (events not coming through >>> on Macs etc) and while I think we are safe on Linux, I am not sure we want >>> to go with this elsewhere. >>> >> >>> >> 2) inotify brings more entropy into the codebase, it is another thing >>> we need to take care of etc (however, it is all concentrated in one class >>> and pretty much "isolated" from everything else) >>> >> >>> >> I made this feature optional and it is turned off by default so >>> people need to explicitly opt-in into this so we are not forcing it on >>> anybody. >>> >> >>> >> If we do not want to go with inotify, another option would be to have >>> a background thread which would periodically check if a manifest exists on >>> a disk, if it does not, then a snapshot does not either. While this works, >>> what I do not like about this is that the primary reason we moved it to >>> memory was to bypass IO as much as possible yet here we would introduce >>> another check which would go to disk, and this would be done periodically, >>> which beats the whole purpose. If an operator lists snapshots once a week >>> and there is a background check running every 10 minutes (for example), >>> then the cummulative number of IO operations migth be bigger than us just >>> doing nothing at all. For this reason, if we do not want to go with >>> inotify, I would also not implement any automatic background check. Instead >>> of that, there would be SnapshotManagerMbean#refresh() method which would >>> just forcibly reload all snapshots from scratch. I think that manual >>> deletion of snapshots is not something a user would do regularly, snapshots >>> are meant to be removed via nodetool or JMX. If manual removal ever happens >>> then in order to make it synchronized again, the refreshing of these >>> snapshots would be required. There might be an additional flag in nodetool >>> listsnapshots, once specified, refreshing would be done, otherwise not. >>> >> >>> >> How does this all sound to people? >>> >> >>> >> Regards >>> > >>> >>> >>>
