Documentation

Backup

  1. Incremental Backups
    1. Implementation
      1. Approach
      2. Detecting Other Backup Programs
      3. Ensuring Backup Integrity

Reconnect supports full and incremental backups.

Incremental Backups

Incremental backups can be significantly faster than full backups as they only copy modified files from your Psion. These always contain the full set of files on your Psion; unchanged files are copied from the most recent backup. They are safe (and the default behaviour) but it is important to understand the way they work if you plan to use your Psion with other backup programs like PsiWin or FastBackup.

Important

Although Reconnect implements mechanisms to detect when other programs have been used to back up a Psion (potentially invalidating the archive flag state), using incremental backups with other Psion backup programs should be considered unsafe and may result in data loss.

Implementation

Approach

In order to support fast incremental backup, Reconnect relies on the FAT file attribute. This attribute is used by both EPOC16 and EPOC32 (and other many other operating systems) to indicate that a file has been modified and needs to be backed up (‘archived’).

On completion of a successful backup, Reconnect clears the archive attribute for all files that have been backed up, recording that they have been archived. When performing a subsequent incremental backup, it only copies files with the archive attribute set (as a result of being modified).

To ensure Reconnect can correctly identify the files that have changed, it is important that Reconnect and EPOC are the only things that set these archive attributes. If they are cleared by another backup program, Reconnect has no way to know that a file has been modified.

Detecting Other Backup Programs

Reconnect writes a special canary1 file (C:\System\Sync\lastbackup.txt on EPOC32 and M:\sync\lastback.txt on EPOC16). This file contains the identifier of the last successful backup (e.g., 477A420D-84CC-4CCF-97FF-3837425C2F89). It is used to lookup the last backup, and it is used to detect other backup programs.

Unlike every other file, Reconnect never clears the archive attribute of the canary file and, when performing a new incremental backup, it checks that the archive flag is still set. If the archive flag has been cleared, Reconnect performs a full backup. This guard works by assuming that any other backup program is unaware of Reconnect and, if it uses archive flags as part of its backup strategy, clear the archive flag of all files it backups up including, crucially, that of the canary.

Ensuring Backup Integrity

In addition to checking the canary, Reconnect also ensures all unmodified files exist in the most recent backup and that they are the same file size as those reported by the Psion. Only in this scenario is the last backup considered as safe source for these files, and will Reconnect perform an incremental backup.

  1. A mechanism used as an early warning system. This term has its origins in early coal mining where miners would use a canary to detect odorless gas; with smaller lungs, the canary would expire quickly, giving a vital early warning. ↩