CJFS: Concurrent Journaling for Better Scalability.

In this paper, we propose CJFS, Concurrent Journaling Filesystem. CJFS extends EXT4 and addresses the fundamental limitations of the EXT4 journaling design, which are the main cause of the poor scalability of EXT4. The heavy-weight EXT4 journal suffers from two limitations. First, the journal commit is a strictly serial activity. Second, the journal commit uses the original page cache entry, not the copy of it, and subsequently any access to the in-flight page cache entry is blocked. To address these limitations, we propose four techniques, namely Dual Thread Journaling, Multi-version Shadow Paging, Opportunistic Coalescing, and Compound Flush. With Dual Thread design, CJFS can commit a transaction before the preceding journal commit finishes. With Multi-version Shadow Paging, CJFS can be free from the transaction conflict even though there can exist multiple committing transactions. With Opportunistic Coalescing, CJFS can mitigate the transaction lock-up overhead in journal commit so that it can increase the coalescing degree - i.e., the number of system calls associated with a single transaction - of a running transaction. With Compound Flush, CJFS minimizes the number of flush calls. CJFS improves the throughput by 81%, 68% and 125% in filebench varmail, dbench, and OLTP-Insert on MySQL, respectively, against EXT4 by removing the transaction conflict and lock-up overhead.