# Instant restore after a media failure

## TL;DR

This paper introduces instant restore, a technique allowing immediate read/write access to data during media recovery, drastically reducing downtime from hours to seconds with minimal overhead.

## Contribution

The paper presents an implementation of instant restore that enables on-demand data segment restoration, significantly improving recovery times in database systems.

## Key findings

- Transaction latency after failure is reduced to less than a second.
- Overhead of instant restore on normal processing is minimal.
- System availability is increased by several 'nines' using simple software techniques.

## Abstract

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.