Media failures usually leave database systems unavailable for several hours until recovery is complete, especially in applications with large devices and high transaction volume. Previous work introduced a technique called single-pass restore, which increases restore bandwidth and thus substantially decreases time to repair. Instant restore goes further as it permits read write access to any data on a device undergoing restore--even data not yet restored--by restoring individual data segments on demand. Thus, the restore process is guided primarily by the needs of applications, and the observed mean time to repair is effectively reduced from several hours to a few seconds. This paper presents an implementation and evaluation of instant restore. The technique is incrementally implemented on a system starting with the traditional ARIES design for logging and recovery. Experiments show that the transaction latency perceived after a media failure can be cut down to less than a second and that the overhead imposed by the technique on normal processing is minimal. The net effect is that a few "nines" of availability are added to the system using simple and low-overhead software techniques.
Techniques to recover databases from media failures were initially presented in the seminal work of Gray @cite_15 and later incorporated into the ARIES family of recovery algorithms @cite_6 . In ARIES, restore after a media failure first loads a backup image and then applies a redo log scan, similar to the redo scan of restart after a system failure. Fig. illustrates the process, which we now briefly describe. After loading full and incremental backups into the replacement device, a sequential scan is performed on the log archive and each update is replayed on its corresponding page in the buffer pool. A global value (called media recovery redo point'' by @cite_6 ) is maintained on backup devices to determine the begin point of the log scan.
@cite_15 @cite_6
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This paper is a compendium of data base management operating systems folklore. It is an early paper and is still in draft form. It is intended as a set of course notes for a class on data base operating systems. After a brief overview of what a data management system is it focuses on particular issues unique to the transaction management component especially locking and recovery. DB2 TM , IMS, and Tandem TM systems. ARIES is applicable not only to database management systems but also to persistent object-oriented languages, recoverable file systems and transaction-based operating systems. ARIES has been implemented, to varying degrees, in IBM's OS 2 TM Extended Edition Database Manager, DB2, Workstation Data Save Facility VM, Starburst and QuickSilver, and in the University of Wisconsin's EXODUS and Gamma database machine.