When upgrading to a new release of Berkeley DB, it may be necessary to upgrade the on-disk format of already-created database files. Berkeley DB database upgrades are done in place, and so are potentially destructive. This means that if the system crashes during the upgrade procedure, or if the upgrade procedure runs out of disk space, the databases may be left in an inconsistent and unrecoverable state. To guard against failure, the procedures outlined in Upgrading Berkeley DB installations should be carefully followed. If you are not performing catastrophic archival as part of your application upgrade process, you should at least copy your database to archival media, verify that your archival media is error-free and readable, and that copies of your backups are stored offsite!
The actual database upgrade is done using the DB->upgrade method, or by dumping the database using the old version of the Berkeley DB software and reloading it using the current version.
After an upgrade, Berkeley DB applications must be recompiled to use the new Berkeley DB library before they can access an upgraded database. There is no guarantee that applications compiled against previous releases of Berkeley DB will work correctly with an upgraded database format. Nor is there any guarantee that applications compiled against newer releases of Berkeley DB will work correctly with the previous database format. We do guarantee that any archived database may be upgraded using a current Berkeley DB software release and the DB->upgrade method, and there is no need to step-wise upgrade the database using intermediate releases of Berkeley DB. Sites should consider archiving appropriate copies of their application or application sources if they may need to access archived databases without first upgrading them.
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