Configuring Epochs

Epoch configuration impacts how your database recovers from failure, handles historical data, and purges data from disk. Vertica provides epoch management parameters for system-wide epoch configuration. Epoch management functions enable you to make ad hoc adjustments to epoch values.

Historical Query and Recovery Precision

When you execute a historical query, Vertica returns an epoch within the amount of time specified by the EpochMapInterval configuration parameter. For example, when you execute a historical query using the AT TIME time epoch clause, Vertica returns an epoch within the parameter setting. By default, EpochMapInterval is set to 180 seconds. You must set EpochMapInterval to a value greater than or equal to the AdvanceAHMInterval parameter:

=> SELECT node_name, parameter_name, current_value FROM CONFIGURATION_PARAMETERS
     WHERE parameter_name='EpochMapInterval' OR parameter_name='AdvanceAHMInterval';
 node_name |   parameter_name   | current_value
-----------+--------------------+---------------
 ALL       | EpochMapInterval   | 180
 ALL       | AdvanceAHMInterval | 180
(2 rows)

During failure recovery, Vertica uses the EpochMapInterval setting to determine which epoch is reported as the last good epoch (LGE).

History Retention and Purge Workflows

Vertica recommends that you configure your epoch parameters to create a purge policy that determines when deleted data is purged from disk. If you use historical queries often, then you need to find a balance between saving deleted historical data and purging it from disk. An aggressive purge policy increases disk utilization and improves query performance, but also limits your recovery options and narrows the window of data available for historical queries.

There are two strategies to creating a purge policy:

• Set HistoryRetentionTime to specify how long deleted data is saved (in seconds) as an historical reference.
• Set HistoryRetentionEpochs to specify the number of historical epochs to save.

See Setting a Purge Policy for details about configuring each workflow.

Setting HistoryRetentionTime is the preferred method for creating a purge policy. By default, Vertica sets this value to 0, so the AHM is 1 less than the current epoch when the database is running properly. You cannot execute historical queries on epochs that precede the AHM, so you might want to adjust this setting to save more data between the present time and the AHM. Another reason to adjust this parameter is if you use the Roll Back Database to Last Good Epoch option for manual roll backs. For example, the following command sets HistoryRetentionTime to 1 day (in seconds) to provide a wider range of epoch roll back options:

=> ALTER DATABASE vmart SET HistoryRetentionTime = 86400;

Vertica checks the status of your retention settings using the AdvanceAHMInterval setting and advances the AHM as necessary. After the AHM advances, any deleted data in an epoch that precedes the AHM is purged automatically by the Tuple Mover mergeout process.

If you want to disable any purge policy and preserve all historical data, set both HistoryRetentionTime and HistoryRetentionEpochs to -1:

=> ALTER DABABASE vmart SET HistoryRetentionTime = -1;
=> ALTER DATABASE vmart SET HistoryRetentionEpochs = -1;

If you do not set a purge policy, you can use epoch management functions to adjust the AHM to manually purge deleted data as needed. Manual purges are useful if you need to update or delete data uploaded by mistake. See Manually Purging Data for details.