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When partition reassignment removes topic partitions from a offline broker, those removed partitions become orphan partitions to the offline broker. When the offline broker comes back online, it is not able to clean up both data and folders that belong to orphan partitions.  Log manager will scan all log dirs during startup, but the time based The retention policy on a topic partition will not be kicked out until the replicaHighWatermark  has been set. replicaHighWatermark of a partition is set when known to the broker becomes as either leader partition or follower of the partition (i.e., until "replicaHighWatermark" is set).  Orphan partitions will never have chance to get replicaHighWatermark setnot become leader or follower partitions and thus will not have retention.  In addition, we do not have logic to delete folders that belong to orphan partition today.  This KIP provides a mechanism for brokers to remove orphan partitions automatically. 

Public Interfaces

• Add two metrics: 
  1) kafka.log:type=LogManager,name=OrphanLogPartitionCount  ( 
  type = gauge
  value = the number of orphan partitions) and
  2) kafka.log:type=LogManager,name=OrphanLogPartitionSize (
  type = gauge
  value = the size of orphan partitions)

Proposed Changes

a) Provide a mechanism to remove orphan partition automatically. The   The orphan partitions removal works in three phases. as follows:

1. Collect orphan partitions.
   The orphan partition set will be determined based on the first leaderandISR request when a broker starts. Topic Partitions that are present in the broker but not in the leaderandISR request are determined as orphan partitions. 
   
   
2. Schedule the orphan partition removal
   Schedule the orphan partition removal after a certain time interval
3. Initialize phase
   During a broker startup, broker calculates the initial set of orphan partitions based on the partition information from the first leaderandISR request. 
4.  timeout/correction phase (such as 2 hours of timeout, defined by an internal timer)
   During timeout phase, a broker updates its knowledge about partitions over time. The firstleaderandISR request the broker receives might be outdated (due to dual controllers, network partitions, etc.). However, during the timeout phase , the broker will receive more leaderandISR requests and use partitions information from leaderandISR requests to remove partitions that the broker is responsible for from the initial orphan partition set.   We rely on the fact that, until timeout timer expires, there is at least one valid leaderandISR for any given partition hosted by the broker. 
   internally in the broker). This time interval enables the broker to revive orphan partitions for pending partition reassignment in the cluster. Since partition reassignment request might come as separated leaderandISR request, the delayed orphan partition removal makes it possible for the broker to re-host the same topic partition without actually fetching entire toptic partition data from partition leader. 
   
   
5. Remove the orphan partitions.
   Deletion phase.
   The broker removes orphan partitions (including partition folders) whose log segments are all older than the broker default retention period.  Broker will not distinguish between the log compacted topic and time-retention topics for those partitions in orphan partition set. The default retention period of a broker is used for all orphan partitions. Broker only removes orphan partitions whose log segments are all older than the default retention period. This is to ensure broker will not try to delete new data. If some orphan partitions cannot be removed immediately because the retention period has not been reached, a new deletion will be scheduled again to perform deletion.

b) Adding metrics to keep track of the number of orphan partitions and the size of these orphan partitions . 

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as described in the public Interface section. 

Compatibility, Deprecation, and Migration Plan

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