Retention lock is an optional feature to safeguard your protected database backups from inadvertent changes or malicious damages, such as ransomware attacks.

Retention lock applies to the backup retention period defined in a protection policy. Recovery Service mandates a minimum delay of 14-days for the retention lock to take effect. During the scheduled delay, you can either increase or decrease the backup retention period or disable the retention lock, if necessary.

After the scheduled delay ends, the retention period is permanently locked. You are only allowed to increase the retention period. Recovery Service prevents the modification or deletion of backups until the backup retention period ends. For example, assume that a custom protection policy retains backups for 50 days. When the retention lock is in effect, you are only allowed to increase the backup retention period to a maximum 95 days, and Recovery Service prohibits the deletion of protected database backups during the 50 day retention period.

See, Using Retention Lock to Protect Backups for additional information.