CJFS: Concurrent Journaling for Better Scalability

In this paper, we propose CJFS, \emph{Concurrent Journaling Filesystem}. CJFS extends EXT4 filesystem and addresses the fundamental limitations of the EXT4 journaling design, which are the main cause for the poor scalability of EXT4 filesystem. 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.