Pulsar 面向 segment 資料塊的架構允許 topic backlog 增長到十分巨大，如果有沒加以限制，隨著時間的增加，管理與使用的代價將越來越高。
有一個減輕這個問題的辦法，那就是使用分層存儲。 With tiered storage, older messages in the backlog can be moved from bookkeeper to a cheaper storage mechanism, while still allowing clients to access the backlog as if nothing had changed.
Data written to bookkeeper is replicated to 3 physical machines by default. However, once a segment is sealed in bookkeeper is becomes immutable and can be copied to long term storage. 長期存儲可以達到節省存儲成本的目的，通過使用如 Reed-Solomon error correction 的機制，只需更少的物理數據備份。
Pulsar currently supports S3 as a long term store. Offloading to S3 triggered via a Rest API or command line interface. The user passes in the amount of topic data they wish to retain on bookkeeper, and the broker will copy the backlog data to S3. The original data will then be deleted from bookkeeper after a configured delay (4 hours by default).
搭建分層存儲的詳細資訊，請參考 Tiered storage cookbook。