Data logging: a method for efficient data updates in constantly active RAIDs

RAIDs (Redundant Arrays of Independent Disks) are a set of disks organized to achieve parallel I/O to multiple disks and to provide tolerance of disk failures. RAIDs offer these advantages at the cost of additional space and additional disk I/O for writes. Previous methods of reducing this I/O overhead suffered from such problems as requiring periods during which data is reorganized and not available, destroying the physical locality of data, or weakening the RAID's fault-tolerance properties. We propose a new method called data logging which reduces the I/O overhead without requiring periodic downtime for reorganization. Instead, incremental maintenance can be performed concurrently with routine processing. This is particularly advantageous in applications requiring "24/spl times/7" uptime. Data logging preserves both physical locality of data and RAID fault tolerance. The major cost of our method is a moderate amount of nonvolatile RAM. This paper describes our method, as well as two schemes for efficient encoding of the information that must be stored in nonvolatile RAM.