Currently in Trove, we support full offline backups for DB2 which is the default backup mechanism for DB2. This enables users to take full backups of DB2 databases when no applications are connected to or using these databases. While useful for cases where this downtime is okay, it is not very useful in a production type environment were applications are expected to be online all the time. DB2 provides a way to do online backups and this spec outlines how we can implement a full online backup mechanism in Trove.
Launchpad Blueprint: https://blueprints.launchpad.net/trove/+spec/db2-online-backup
There are two types of backups we can perform using DB2 - offline and online. Offline backups require users and applications to be disconnected from the databases thereby requiring a certain down time. Online backups on the other hand, can be performed while users are still connected to databases. Trove currently supports full offline backups for DB2. In this spec, we explain how to enable full online backups for DB2 in Trove.
As mentioned above, the difference between offline and online backup is that in the former case all applications must disconnect from the database while in the latter, applications can still be connected to databases while backup is being taken. In order to enable online backups, archive logging needs to be enabled. This allows applications to use the database even while backups are being taken. Any updates made to the database is recorded in the logs.
Logging is a mechanism where every updates to a database is recorded to transaction logs (also called active logs) as the data is being modified. There are 2 types of logging mechanism - circular and archive logging. By default, circular logging is enabled which means that after the last active log is filled, the first active log is overwritten. With circular logging as the default logging mechanism, only offline backups can be taken.
Using archive logging, the logs are stored in sequence and as logs get full, they are archived. Archive logging allows users to restore databases by roll forwarding to a particular point in time or a point before failure. Hence, users can restore a database from a backup image and roll forward to a particular point using logs thus getting it to a consistent state. For an introduction to archive logging, please refer  and for an overview of how transactional logging works in DB2, please refer .
To enable full online backup for Trove, the following steps need to be taken:
Create a directory to store the backups and archive logs - /home/db2inst1/db2inst1/backup/ArchiveLogs
When a database is created, if online backups is configured for DB2(through the backup_strategy config property), then configure the following database configuration parameter: ‘LOGARCHMETH1’ to point to a location where the archive logs can be stored using the command:
db2 update database configuration for <db> using LOGARCHMETH1 ‘DISK:/home/db2inst1/backup/ArchiveLogs’
Setting this database configuration parameter enables archive logging and stores the archive logs in a location different from the primary/active logs.Once this change is made, a full offline backup needs to be taken before any connections can be made to the database (this is because the database goes into a ‘BACKUP_PENDING’ state when this configuration change is made).
db2 backup database <db> to /home/db2inst1/db2inst1/backup
This backup can be deleted from the backup directory after this operation.
Check if archive logging is enabled for each database
Check if there is enough space to store backups (which will include database and archived logs). The get_dbsize_info function that DB2 provides can be used to get the size of the database directory. We can query the SYSIBMADM.LOG_UTILIZATION table to get the log space used for each database.
Do an online backup of the database and include the archive logs along with the backup image itself:
db2 backup database <db> ONLINE to /home/db2inst1/db2inst1/backup INCLUDE LOGS
Once the backup is complete, compress/encrypt the backup inside the volume storage.
The compressed/encrypted file can then be stored in Swift under the database_backups container.
Once the backups are stored in Swift, we can go ahead and delete the backup files and archive logs from the backup directory.
Restoring a database includes restoring from an online backup image and rolling forward the logs to the end of the backup to make sure the database is in a consistent state.
Create a directory where the backup files and archive logs can be stored
Retrieve the backup from storage to the proper location
Use the following commands to restore the database and ensure it is in a consistent state: - db2 RESTORE DATABASE <db> FROM <backup_dir> LOGTARGET <log_dir>
The above command restores the database from the specified online backup directory and extracts the archive log files into the log_dir
This command will roll forward the restored database to a consistent state by applying the archive logs. To see more details on the ROLLFORWARD command, please refer .
The default configuration values of the following will need to change to correspond to the respective DB2 locations.
The DB2 guestagent will be modified to add support for full online backup and restore functionality. In addition, new strategies will also be added to support these new features.
and the following new strategy files will be modified to add a new class for online backups:
New classes will be added for online backup strategy and the existing class needs to be renamed from DB2Backup -> DB2OfflineBackup
Mariam John (johnma)
There will only be one work item for this feature. This includes implementing the strategies for DB2 online backup and restore and make the necessary guestagent changes. This will also include test-cases necessary to test the new functionalities.
Add new test cases to test online backup and restore functionality for DB2:
- Add new unit tests to test the newly implemented functionality
- Add a new DB2 helper class to the existing integration-test framework.
The datastore documentation should be updated to reflect the enabled features.