[Druid 0.9.0] Question regarding druid merge task

I read about the merge task here http://druid.io/docs/latest/ingestion/tasks.html and tried it. Based on my experience so far I have to make a json which is dependent on actual segments present. Is there something present which is equivalent of saying “Hey druid go and keep on merging segments into day granularity for datasource1 for all segments which are more than 1 month old”

there the option to activate auto merge based on segment size threshold.

First set it to rue :

druid.coordinator.merge.on
Boolean flag for whether or not the coordinator should try and merge small segments into a more optimal segment size.

druid.coordinator.period.indexingPeriod
How often to send indexing tasks to the indexing service. Only applies if merge or conversion is turned on.

finally you can override the default size via dynamic config

http://druid.io/docs/latest/configuration/coordinator.html#dynamic-configuration

The relevant one for size is mergeBytesLimit I guess? Why is that turned off by default? Is it not reliable?

Also doing this will make changes in historical node/deep storage and metastore by itself. Correct?

it is off by default because it introduces more load and not necessary for all the use cases.

It is used in production by multiple companies so yes it is reliable.
generally you want to set the right targetPartitionSize that will produce the right segment size hence you don’t need this.

yes you need to set the period of re-indexing and then the mergeBytesLimit.

Did not understand this part

generally you want to set the right targetPartitionSize that will produce the right segment size hence you don’t need this.

What is that I don’t need? Can you please elaborate?

I did this, the task got and the segment got merged. But the old segments as well as new segments have used = true. So what did it do? Does it take time to do that?

But the old segments are still there. I looked into druid_segments table and the used flag is true for both old and new segment.