Information residing in stable storage is never lost. To implement such storage, we need to replicate the required information on multiple storage devices (usually disks) with independent failure modes. We also need to coordinate the writing of updates in a way that guarantees that a failure during an update will not leave all the copies in a damaged state and that, when we are recovering from a failure, we can force all copies to a consistent and correct value, even if another failure occurs during the recovery
A disk write results in one of three outcomes:
- Successful completion. The data were written correctly on disk
- Partial failure. A failure occurred in the midst of transfer, so only some of the sectors were written with the new data, and the sector being written during the failure may have been corrupted
- Total failure. The failure occurred before the disk write started, so the previous data values on the disk remain intact
