Force merge should be cancellable

[nik9000]

We have the infrastructure to make long running stuff cancellable. Force merge seems like something that you might want to cancel.

Cancel doesn't have to immediately cancel (can't/shouldn't kill threads in Java), just make a reasonably good effort to cancel the task.

[s1monw]

how would that work? you can't just go and stop the merge once it's kicked off - I think having an API to do that is misleading and will be disappointing to users

[nik9000]

how would that work? you can't just go and stop the merge once it's kicked off - I think having an API to do that is misleading and will be disappointing to users

That is why I made this a discussion! I haven't read much of that code but it feels like an "obvious" use for cancellable tasks. If it is hard to impossible at least we'll have this issue we can point people to when they ask for it.

[pickypg]

Perhaps it can short circuit? Since force merge is actually a series of merges in the background, maybe it can just stop in the middle if possible? Then perhaps we can augment the API to state where it is so that it can succeed / fail?

[kuipertan]

I need this api.

Once start it , it takes long time to finish the merging work.

[geekpete]

So it could finish the current segment merges but not start any new segment merges?
When segments are merged, the new ones are written out and when confirmed as completed, the old ones are then removed. This is why the disk usage grows then shrinks again. So a way to cancel the current/in-progress merges and reinstate the existing segments that were about to be thrown away seems like what you'd want it to do?

And managing it with the tasks api would be nice.

[elasticmachine]

Pinging @elastic/es-core-infra

[tomcallahan]

I'm afraid we don't have the underlying infrastructure to be able to accomplish this - see #15975