Managing Ephemeral Configuration Database Space

Junos devices maintain versioning for ephemeral database objects, and as a result, the database also retains and stores deleted objects. A deletion is characterized by:

• Explicitly deleting the configuration.

• Changing the value of a configuration attribute.

• Reordering elements during a load update operation.

Cyclic versioning reclaims the space occupied by objects that were deleted in a previous version of the database. The cyclic version value determines the ephemeral database version in which the system reclaims deleted objects during a commit operation. The default cyclic version value for each ephemeral database instance is 10. Thus, on devices that support cyclic versioning, the system, by default, reclaims the space occupied by deleted configuration objects with each commit. You can modify the setting on a per-instance basis. To disable cyclic versioning, set the cyclic version value to 0.

For example, if you use the default cyclic version value of 10, then:

• After the 11th commit (version 11), the device reclaims the space occupied by objects that were deleted in version 1.

• After the 12th commit (version 12), the device reclaims the space occupied by objects that were deleted in version 2.

• After the 13th commit (version 13), the device reclaims the space occupied by objects that were deleted in version 3.

This process continues with each subsequent commit operation. As illustrated in the previous example, the version from which the system reclaims deleted objects during the current commit operation is:

version to reclaim = current version - cyclic version

In earlier releases and on devices that do not use cyclic versioning, the ephemeral database default behavior is to purge the database when it reaches the maximum allowable version. A purge operation reclaims the space used by deleted objects but requires all processes to read the full configuration. A database purge operation involves:

• Creating a new database.

• Copying only the active configuration objects from the current database into the new database.

• Setting the version for all active configuration objects in the new database to version 1.

Resizing an ephemeral database might be necessary if cyclic versioning is enabled and you make frequent changes to the database that involve deleting or reordering elements. On devices that support cyclic versioning, the system automatically reclaims the space occupied by deleted objects during a commit operation. However, the system might or might not reallocate the freed space for new configuration objects when you update the database. If the system does not reallocate the space, then the database can become fragmented over time. Resizing an ephemeral database reclaims the space occupied by all deleted objects and defragments the database, which can improve performance.

A database resize operation involves:

• Creating a new database.

• Copying only the active configuration objects from the current database into the new database.

• Setting the version for all active configuration objects in the new database to the latest version.

As with the static configuration database, you can configure Junos devices to resize the ephemeral configuration database. After you configure database resizing, Junos devices resize the ephemeral database during a commit operation if the database's space exceeds the specified thresholds. You can modify the thresholds for each ephemeral instance.

The system resizes the database when the database size meets the criteria for both of the following configuration statements:

• database-size-diff—Minimum difference between the database size and the actual usage. Default is 100 MB.

• database-size-on-disk—Minimum configuration database size on disk. Default is 450 MB.

For example, if you configure the device to use the default values, then the system resizes the database when the database size on disk exceeds 450 MB and the database size is 100 MB greater than the actual database usage.

For information about configuring database resizing, see Resize an Ephemeral Database Instance.

Use the show system configuration database usage command to display the database's disk space usage. The command displays the current database size on disk, the actual database usage, and the maximum size of the database.

user@host> show system configuration database usage ephemeral-instance default
Maximum size of the database: 692.49 MB
Current database size on disk: 1.50 MB
Actual database usage: 1.49 MB
Available database space: 691.01 MB