What to Do If Your Backup Gets Corrupted

Sheet happens. In the last lecture, we also went through the step-by-step process of fully restoring a database after a crash. But, you know, real life loves to throw curveballs. What if your backup turns out to be corrupted? Well, it's time to figure this out.

Where's the best place to put your resume?

Just kidding. We're gonna dive into the technical stuff now. But remember, every joke has a grain of truth :)

Analyzing Backup Corruption

A corrupted backup is like your thesis file that just won't open right before the deadline. The symptoms are pretty similar:

• Errors when trying to restore: when running pg_restore you might see errors like:

  pg_restore: [archiver] input file does not appear to be a valid archive

• Some data is missing or the file has the wrong size (like, suddenly tiny or even zero).
• The unpacked SQL file has data cut off, weird symbols, or empty sections.

Backups can get corrupted for all sorts of reasons:

• Backup process interrupted (like a power outage or the system shutting down).
• Bad file transfer (errors copying to a flash drive or over the network).
• Hardware failures (disks, RAM, RAID array issues).
• Mistakes in system or backup utility configuration.
• Viruses, malware, and of course, the classic "oops, wrong click".

Recovering Data from a Corrupted Backup

If the file's corrupted, it's not the end of the world. Sometimes you can still get something back. Let's check out a few approaches.

Trying to Restore with pg_restore

If the file was created with pg_dump in custom or directory format, try restoring with pg_restore and the --ignore-errors flag. Example command:

pg_restore --dbname=your_database --ignore-errors backup_file.dump

This flag tells the utility to ignore errors during restore and keep going. Sure, you'll lose data where the corruption is, but you might get at least some info back.

If this works, you should carefully check the restored data after the process and note what's missing.

Using Partial Data

Say you have a backup in plain SQL text format. Try opening it in a text editor (preferably something fancy, like Visual Studio Code or Notepad++) to see where it cuts off. If the file is readable and most SQL queries are intact:

1. Delete the corrupted part of the file.
2. Run the remaining SQL queries manually or with psql:

psql -U username -d database_name -f partial_backup.sql

Reading a custom Format File

If your backup is in custom format, you can try to extract its contents piece by piece:

pg_restore --list backup_file.dump > file_list.txt

This command will create a list of all objects in the backup. Then you can try restoring individual items (like tables or schemas) with:

pg_restore --dbname=your_database --use-list=file_list.txt backup_file.dump

By editing file_list.txt, you can skip corrupted items during restore.

Trying to Restore via Archive Logs (WAL)

If you're using incremental or differential backups (like with pg_basebackup), you probably have archive logs (WAL files). They store all changes since the last backup. To restore your data, you can:

1. Find the last complete backup (like a full backup).
2. Tell PostgreSQL where the WAL files are:

   restore_command = 'cp /path/to/wal_directory/%f %p'

3. Run the restore.

Using Third-Party Tools

Sometimes you can minimize backup corruption with file recovery utilities. A popular tool is ddrescue. Example command:

ddrescue --force backup_file.dump recovered_dump.file

This tool will try to recover as much data as possible and make a new copy of the file.

Preventing Backup Corruption

Yeah, dealing with corrupted files is stressful. Let's think about how to avoid this stuff.

Storing Copies in Multiple Places

Just like in any backup project, always follow the rule "don't put all your eggs in one basket". You should duplicate your backups:

• On the local server.
• In the cloud (Amazon S3, Google Cloud Storage, or even something simple like Dropbox).
• On external drives (if you wanna be super safe).

Regularly Checking Backup Integrity

There's a saying in IT: "A backup that hasn't been tested isn't a backup." Regularly check your backups by doing test restores. To do this:

1. Upload the backup to a test server.
2. Restore data from it.
3. Check if everything's correct.

Using Checksums

When making backups, generate file checksums (like with MD5 or SHA256):

md5sum backup_file.dump > backup_file.md5

When restoring, compare the current checksum to the original:

md5sum -c backup_file.md5

This way you'll know ahead of time if the file's corrupted.

Examples of Restoring from Corrupted Backups

Let's look at a couple of real-life cases.

Case 1. File Transfer Error

You're transferring a backup via FTP, but suddenly notice the file is smaller than expected. You used pg_dump in text format, so:

1. Opened the file in an editor.
2. Deleted the corrupted part of the data.
3. Restored the working part of the file with psql.

Case 2. Partial Data Loss in WAL

Your server used incremental backups and archived WAL files. Suddenly, some files disappeared. But you managed to restore data using pg_basebackup and the remaining WAL files, pointing to their path in the recovery settings.

Remember, backup isn't just about making copies, it's about checking them too. Don't wait for the apocalypse to find out your backups are useless. And if your backup does get corrupted, don't panic: PostgreSQL gives you plenty of ways to get your data back. The main thing is to act fast and carefully!

And hey, if nothing worked—go back and reread the first point of this lecture.