Consistency of Databases on Commodity Disk Drives

Most database systems use ARIES-like logging and recovery scheme to recover from failures and guarantee transactional consistency. ARIES relies on the Write-Ahead Logging (WAL) protocol which requires that log records be written durably prior to the corresponding data changes. In order to enforce WAL, database systems rely on the write-through capability of the storage media. While SCSI disks that are commonly deployed in enterprise servers support write-through, commodity hard drives do not. In the past, database systems were mostly limited to enterprise servers; however, today they are being heavily deployed in large-scale internet services and personal information management systems. In order to minimize costs, these systems use commodity hard drives that have controller caches and lack write-through. These drives delay and reorder the writes thereby breaking the WAL protocol recovery is based on. Here we present a solution to enforce WAL and guarantee recoverability of the database on these drives. We also present performance measurements demonstrating that our approach does not adversely impact transaction throughput.