Configuring Log Backups

You have to understand the SAP HANA automatic log backup mechanism, the different log modes, and need to be able to configure the log backups according to the backup and recovery strategy.

You can enable or disable automatic log backup with the enable_auto_log_backup parameter. The default setting is enable_auto_log_backup = yes.

To allow the reuse of log segments, the system performs automatic log backups. During a log backup, the log segments are copied from the basepath_logbackup location to the location basepath_logbackup.

A log segment is automatically backed up in the following situations:

• The log segment is full.

• The log segment is closed after exceeding the configured timeout threshold.

• The database is started.

Log segments can only be overwritten by the system after they have been backed up. If no regular log backups take place, you risk that the file system basepath_logvolume becoming full.

To avoid the location basepath_logbackup from becoming full, make sure to archive the log backups according to your backup cycle defined in your backup and recovery strategy.

The system can reuse the space that is occupied in the log volume by the log segments. The log_mode parameter controls how the log segments are reused.

The configuration log_mode = normal means log segments are freed as soon as a log backup is performed. This is the default log mode and is required for all systems that need to able to be recovered.

With log_mode = normal a point-in-time recovery, a full recovery, and recover to a specific data backup are possible.

log_mode = normal is the default mode and support the following features:

• Keeps log segments until back up

• Log backups are performed automatically when a log segment is full, or when a log segment becomes older than 15 minutes. The log backup timeout can be configured using the parameter log_backup_timeout_s.

• Log backup directory configured with parameter basepath_logbackup

• The backup catalog is maintained.

• Restoring from any available data backup with log replay to the last committed state.

• Restoring any available data backup without log replay.

Log segments in the log area are only released for overwrite after a successful log backup. In some situations, there can be a delay in releasing log segments because they are waiting to be backed up. As a result of this delay, the log area can grow. If the log segments cannot be backed up and released faster than SAP HANA will automatically create a new log segment, this way the can continue to work by adding new log segments until the log volume area become full.

For productive systems, use normal log mode because it provides the highest security for the restoration of data during a recovery of the SAP HANA database. In normal log mode, the system automatically creates log backups that can be used for a recovery in addition to the data backups. However, more backup space is required in this log mode because of the log backups. Therefore, an operational concept for administrating data and log backups is a prerequisite for using normal log mode.

After changing the log mode parameters, restart the database system to activate the changes. Also, create a full data backup of the database.

Using this configuration means log segments are freed by savepoints, and no log backups need to be performed. This log mode can be useful, for example, for test installations or training systems that do not need to be recovered.

With log_mode = overwrite, no point-in-time recovery is possible as there are no log backups created. For recovery, only a complete data backup can be used. The only option available for recovery is Recover to a specific data backup.

The parameter log_backup_timeout_s in the global.ini configuration file defines the interval at which log backups are created. By default, the log backup interval is 15 minutes (900 s).

Specifying an appropriate interval for log backups enables you to recover an SAP HANA database with a good Recovery Point Objective (RPO). In the event of database failure, the RPO is the maximum time span of data that will be lost if the log area cannot be used for recovery, and if only data backups, delta backups, and log backups are available.

To provide full point-in-time recoverability, enable automatic log backups in production systems. An alert notifies administrators when automatic log backups are disabled.

With a well-operated SAP HANA system, many log backups are created. It is common to have over 2,000 backup files per day. External backup tools can have problems with digesting so many files. SAP HANA uses several options to reduce the number of backup files that are created.

To improve the performance of log backups, SAP HANA can write all the log segments of a service that are ready to be backed up at a particular time to a single log backup. You can define the maximum size of this single log backup. This option is supported for both file-based backups and third-party tools.

The parameter log_backup_interval_mode defines when a log backup should be triggered.

log_backup_interval_mode = immediate

In the mode "immediate" a log backup will be executed immediately after a log segment becomes full or no later than the service-specific timeout has been reached for a log segment.

log_backup_interval_mode = service

In the mode "service" a log backup will be executed only after a service-specific timeout has been reached. This backup will enclose all log segments in states "writing", "closed" and "truncated".

The maximum size of the log segments to be processed by a single backup operation is defined by the parameter max_log_backup_size in the backup section of the global.ini file. The default value is 16 GB.