rvashishth commented on issue #7500:
URL: https://github.com/apache/pulsar/issues/7500#issuecomment-660245009


   > I thought that the retention policy and backlog quota would control the 
size limit of acknowledged messages and unacknowledged messages independently.
   
   @hozumi I was thinking the same, this might be the reason for whole 
confusion.  I assume backlogQuota and subscription/message backlog are two 
different things, where message backlog is just a cursor and backlogQuota is 
actual storage of unacked message backlog. 
   
   https://pulsar.apache.org/docs/en/cookbooks-retention-expiry/#backlog-quotas
   > Backlogs are sets of unacknowledged messages for a topic that have been 
stored by bookies. Pulsar **stores all unacknowledged messages in backlogs** 
until they are processed and acknowledged.
   
   @addisonj you mentioned _Beyond subscriptions, topics themselves just have 
retention_. Is this backlogQuota per topic is just a pointer in size for all 
subscriptions, which mark unacked messages beyond the size threshold as acked? 


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