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WAL-G for Greenplum

You can use WAL-G as a tool for making encrypted, compressed physical Greenplum backups and push/fetch them to/from the remote storage without saving it on your filesystem.

Configuration

WAL-G for Greenplum understands the basic configuration options that are supported by the WAL-G for Postgres, except the advanced features such as delta backups, remote backups, catchup backup, etc.

To configure the backups, the user needs to do two things on each segment host:

  1. Create the configuration file
  2. Configure the WAL archiving

Configuration file

Unlike the WAL-G for Postgres, a config file is a must. It must be placed in the same location on each cluster host, for example, /etc/wal-g/wal-g.yaml. Sample configuration file:

~ # cat /etc/wal-g/wal-g.yaml
PGDATABASE: "postgres"
WALE_S3_PREFIX: "s3://some/s3/prefix/"
WALG_NETWORK_RATE_LIMIT: 8388608
PGPASSFILE: "/home/gpadmin/.pgpass"
WALG_GP_LOGS_DIR: "/var/log/greenplum"
AWS_ACCESS_KEY_ID: "aws_access_key_id"
WALG_UPLOAD_CONCURRENCY: 5
WALG_PGP_KEY_PATH: "/path/to/PGP_KEY"
WALG_DOWNLOAD_CONCURRENCY: 5
WALG_DOWNLOAD_FILE_RETRIES: 15
WALE_GPG_KEY_ID: "gpg_key_id"
WALG_DISK_RATE_LIMIT: 167772160
PGUSER: "gpadmin"
GOMAXPROCS: 6
PGHOST: "localhost"
AWS_ENDPOINT: "https://s3-endpoint.host.name"
AWS_SECRET_ACCESS_KEY: "aws_secret_access_key"
WALG_COMPRESSION_METHOD: "brotli"

WAL archiving

Also, WAL archiving must be configured on each segment primary. Archive command must contain the --content-id flag with the content ID of the segment.

$ vim postgresql.conf
…
wal_level = archive
archive_mode = on	
archive_command = '/usr/bin/wal-g seg wal-push %p --content-id=-1 --config /etc/wal-g/wal-g.yaml'
…

Usage

backup-push

After the successful configuration, use the backup-push command from the coordinator host to create a new backup.

wal-g backup-push --config=/path/to/config.yaml

Delta backups (work in progress)

  • WALG_DELTA_MAX_STEPS

Delta-backup is the difference between previously taken backup and present state. WALG_DELTA_MAX_STEPS determines how many delta backups can be between full backups. Defaults to 0. Restoration process will automatically fetch all necessary deltas and base backup and compose valid restored backup (you still need WALs after start of last backup to restore consistent cluster).

Delta computation is based on ModTime of file system and LSN number of pages in datafiles for heap relations and on ModCount + EOF combination for AO/AOCS relations.

Create delta from specific backup

When creating delta backup (WALG_DELTA_MAX_STEPS > 0), WAL-G uses the latest backup as the base by default. This behaviour can be changed via following flags:

  • --delta-from-name flag or WALG_DELTA_FROM_NAME environment variable to choose the backup with specified name as the base for the delta backup

  • --delta-from-user-data flag or WALG_DELTA_FROM_USER_DATA environment variable to choose the backup with specified user data as the base for the delta backup

Examples:

wal-g backup-push --delta-from-name backup_name --config=/path/to/config.yaml
wal-g backup-push --delta-from-user-data "{ \"x\": [3], \"y\": 4 }" --config=/path/to/config.yaml

backup-fetch

When fetching base backups, the user should pass in the cluster restore configuration and the name of the backup.

wal-g backup-fetch backup_20211202T011501Z --restore-config=/path/to/restore_cfg.json --config=/path/to/config.yaml

WAL-G can also fetch the latest backup:

wal-g backup-fetch LATEST --restore-config=/path/to/restore_cfg.json --config=/path/to/config.yaml

Cluster restore configuration declares destination host, directory, and port for each segment. Sample restore configuration:

{
        "segments": {
                "-1": {
                        "hostname": "gp6master",
                        "port": 5432,
                        "data_dir": "/gpdata/master/gpseg-1"
                },
                "0": {
                        "hostname": "gp6segment1",
                        "port": 6000,
                        "data_dir": "/gpdata/primary/gpseg0"
                },
                "1": {
                        "hostname": "gp6segment1",
                        "port": 6001,
                        "data_dir": "/gpdata/primary/gpseg1"
                }
        }
}

WAL-G can fetch the backup with specific UserData (stored in backup metadata) using the --target-user-data flag or WALG_FETCH_TARGET_USER_DATA variable:

wal-g backup-fetch --target-user-data "{ \"x\": [3], \"y\": 4 }" --restore-config=/path/to/restore_config.json --config=/path/to/config.yaml

WAL-G can fetch the backup onto the specific restore point using the --restore-point flag:

wal-g backup-fetch [OPTIONAL_BACKUP_NAME] --restore-point restore_point_name --restore-config=/path/to/restore_config.json --config=/path/to/config.yaml
  • If backup name is specified, WAL-G will also check if the requested restore point is created after the backup end timestamp.
  • If backup name is not specified, WAL-G will choose the closest backup to the restore point.

WAL-G can fetch the backup onto the restore point closest to the specific time using the --restore-point-ts flag:

wal-g backup-fetch [OPTIONAL_BACKUP_NAME] --restore-point-ts "2022-07-05T01:01:50Z" --restore-config=/path/to/restore_config.json --config=/path/to/config.yaml

Partial restore

--content-ids flag allows to perform the fetch operations only on some specific segments. This might be useful when the backup-fetch operation is completed successfully on all segments except the few ones so the DBA or script can semi-automatically complete the failed backup fetch. For example:

wal-g backup-fetch LATEST --content-ids=3,5,7 --restore-config=restore-config.json --config=/etc/wal-g/wal-g.yaml

Backup fetch mode

--mode allows to specify the desired mode of the backup-fetching.

  • default will do the backup unpacking and prepare the configs [unpack+prepare]
  • unpack will do backup unpacking only
  • prepare will perform config preparation only.
wal-g backup-fetch LATEST --mode=unpack --restore-config=restore-config.json --config=/etc/wal-g/wal-g.yaml

Restore only specific databases

During partial restore wal-g restores only specified databases' files. Use 'database', 'database/namespace.table' or 'database/namespace/table' as a parameter ('public' namespace can be omitted).

Require files metadata with database names data, which is automatically collected during local backup. With remote backup this option does not work.

Restores system databases, tables and aoseg tables automatically.

wal-g backup-fetch LATEST --restore-only=db, "db with spaces" --restore-config=restore-config.json --config=/etc/wal-g/wal-g.yaml

Note: Double quotes are only needed to insert spaces and will be ignored

Example:

--restore-only=my_db,"another db"

is equivalent to

--restore-only=my_db,another" "db

or even

--restore-only=my_db,anoth"e"r" "d"b"

Because of unrestored databases' remains are still in system tables, it is recommended to drop them.

In-place restore

WAL-G can also do in-place backup restoration without the restore config. It might be useful when restoring to the same hosts that were used to make a backup:

wal-g backup-fetch LATEST --in-place --config=/path/to/config.yaml

Delete concurrency

During the delete execution, WAL-G can process segments in parallel mode. To control, how many segments will be processed simultaneously, use the WALG_GP_DELETE_CONCURRENCY setting. The default value is 1.

AO/AOCS size threshold

To control the minimal size of the AO/AOCS segment file to be uploaded into the shared storage, use the WALG_GP_AOSEG_SIZE_THRESHOLD. The higher this value, the bigger the size of a single backup and the smaller the size of the shared AO/AOCS storage folder. Default value is 1048576 (1MB).

AO/AOCS deduplication age limit

To control the maximum possible time starting from the initial upload of the AO/AOCS segment files for their reuse in the following backups, use the WALG_GP_AOSEG_DEDUPLICATION_AGE_LIMIT. Smaller values will result in AO/AOCS files being reuploaded more frequently, leading to larger backups, and vice versa. Default value is 720h (30 days).

restore-point-list

Lists currently available restore points in storage.

Usage:

wal-g restore-point-list [--pretty] [--json]

Check AO/AOCS tables

WAL-G has special command to validate AO/AOCS tables length:

wal-g check-ao-aocs-length

It ensures that table files on disc are not shorter than expected EOF in metadata. To check that last backup has EOF that is less or equal than current EOF, you can add flag:

wal-g check-ao-aocs-length --check-backup

If you want to secect special backup for check, you can add it`s name:

wal-g check-ao-aocs-length --check-backup --backup-name=backup_name