R-Barrier: Rapid Barrier for Software RAID Cache Using Hints from Journaling Filesystem

While adopting cache in software RAID brings performance benefit, it can cause data loss at power failure which results in the broken filesystem consistency. Though I/O Barrier can be used to remove the consistency issue, it sacrifices the write performance of software RAID. To mitigate this tradeoff, we suggest R-Barrier for software RAID. The basic idea of R-Barrier is to avoid the synchronization of the entire cache space. Instead, based on given hints from the filesystem layer, R-Barrier makes software RAID cache select one of the following methods when the cache writes a certain write request into the disk: (1)Strictly ordered destaging for the write requests affecting the filesystem consistency, (2)Reordered destaging for the rest of write requests. We show that R-Barrier guarantees same filesystem consistency with I/O Barrier while the performance degradation of R-Barrier is less than that of I/O Barrier.