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WAL-E needs to be installed on all machines, masters and slaves.

How to install WAL-E

Only one machine, the master, writes WAL segments via continuous archiving. The configuration for the master postgresql.conf is:

archive_mode = on
archive_command = 'envdir /etc/wal-e.d/env wal-e wal-push %p'
archive_timeout = 60

When a master is first setup, a base backup needs to be performed. As the postgres user:

envdir /etc/wal-e.d/env wal-e backup-push $PGDATA

This will perform a base backup and push it to S3. The above archiving commands will stream all deltas in realtime as generated by the Postgres master.

The flow for setting up a new Postgres machine slave, which will be based on a base backup.

1. Install Postgres, configure as a slave.
2. Remove the $PGDATA dir, if it exists. This will be re-created in step 3.
3. Pull a base backup using WAL-E:

envdir /etc/wal-e.d/env wal-e backup-fetch $PGDATA LATEST

4. Implement a $PGDATA/recovery.conf which is configured with WAL-E to replay remote segments:

standby_mode     = 'on'
primary_conninfo = 'host=$HOST user=$USER password=$PASSWORD'
restore_command  = 'envdir /etc/wal-e.d/env wal-e wal-fetch "%f" "%p"'
trigger_file     = '/data/postgresql/9.1/main/trigger'

4b. During recovery, by default WAL-E will attempt to pull and restore all WAL segments available. If you wish to restore to a specific POINT IN TIME than specify

# restore to 4:38PM on 3/6/2012
recovery_target_time = '2012-03-06 16:38:00'

In recovery.conf to recover to that specific point in time. This can be helpful it something bad happened at 4:39PM and you want to restore to 4:38PM.

See Recovery Target Settings for more details.

5. Ensure proper permissions:

chown -R postgres:postgres $PGDATA

6. Start Postgres and tail its log. It should start up and fetch WAL segments from S3, which is being performed by the restore_command in the above recovery.conf.

Setup a CRON to perform base backups on a regular schedule

We have WAL-E configured to push a base backup nightly:

/etc/cron.d/pg_base_backup:

0 8 * * * postgres envdir /etc/wal-e.d/env wal-e backup-push /data/postgresql/9.1/main/