Table of contents

PostgreSQL backups

WAL-E Backups to S3

We use WAL-E to do continuous archiving to an Amazon S3 bucket. PostgreSQL uses a method of archiving transaction logs: this means that transaction logs are copied to a place on disk, and are rotated by a set amount of time or disk usage.

WAL-E uses this function and makes it easy to upload the archived logs to a different location, using environmental variables to specify where the logs should go. It also specifies encryption details.

The archived transactions logs are based upon a “base” backup, which is taken every night. By default we rotate a new transaction log every 5 minutes, so in theory we can recover point in time backups to specific time and dates in this timeframe.

To restore, we can use the commands specified in the WAL-E documentation: backup-fetch and wal-fetch. To make this easier, a script has been written to automate the restore of the very latest backup available (/usr/local/bin/wal-e_restore). The environmental variables which define the AWS credentials will need to be present. After the base backup is imported you can watch postgres restoring WAL chunks by tail -f on the postgres log.

Replication to the standby databases may fail after restoring a backup to the primary. The solution is to also restore the backup to the standby, at which point normal replication will take over. You can also resync it from the primary.

WAL-E failing with errors about GPG

WAL-E does not work with password-protected GPG secret keys, but ours are. If backup restoration is failing with errors about GPG or things not being the expected format (eg, complaining that a file doesn’t have a valid lzo or tar header), the key may be password-protected.

You can edit the key in-place to remove the password:

$ sudo cat /etc/wal-e/env.d/WALE_GPG_KEY_ID
$ sudo -iu postgres gpg --edit-key <key ID>
gpg> passwd
Key is protected.

You need a passphrase to unlock the secret key for
user: <REDACTED>

Enter passphrase: <password from secrets>

Enter the new passphrase for this secret key.

Enter passphrase: <enter>
Repeat passphrase: <enter>

You don't want a passphrase - this is probably a *bad* idea!

Do you really want to do this? (y/N) y

gpg> save

autopostgresqlbackup

This is how PostgreSQL backups have traditionally been taken on the GOV.UK Infrastructure. It is now deprecated. We are continuing to use it as well alongside WAL-E until we are certain that the restores from WAL-E are working ok.

A third-party script called autopostgresqlbackup takes a pg_dump every night and stores them on disk on a dedicated mount point on the PostgreSQL primary machines.

The onsite backup machine (backup-1.management) pulls the latest backup and stores it on disk. Duplicity runs each night to send encrypted backups to an S3 bucket.

To restore from this method:

  • Fetch a backup from either the dedicated mount point, the onsite machine, or the S3 bucket, using duplicity. To decrypt, you may need a password kept in encrypted hieradata.
  • Unzip the file
  • Import into a PostgreSQL primary using psql <dbname> < <file>
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