If you are a SQL DBA then this blog might be a quick refresher for you. I am writing this for Data Protection Manager (DPM) Administrators who are not aware of the cause of the issue. In this blog, we would learn about how to fix error raised by DPM – “The backup failed because of a gap in the LSN the Log files”
One of my client who was not an expert in SQL contacted me for below error which was reported in DPM log.
Description: Last 1 recovery points not created. DPM tried to do a SQL log backup, either as part of a backup job or a recovery to a latest point in time job. The SQL log backup job has detected a discontinuity in the SQL log chain for database since the last backup. All incremental backup jobs will fail until an express full backup runs. For more information, open DPM Administrator Console and review the alert details in the Monitoring task area.
He had no idea about what needs to be done. So, he contacted me for On Demand (55 minutes) services. Even if I didn’t know much about DPM, I took this request and learned something new about DPM.
In SQL Server full recovery model transaction log file gets truncated after transaction log backup. It looks like if the DPM server finds that there is a discontinuity in the log backup and it raises an error. This also means that some other backup application (mostly SQL Server Native Backup) is taking the log backup LSN is not in sequence for backups taken by DPM. If you are an SQLDBA, you can think it as LSN chain break which might happen in log-shipping when we take out-of-band backup.
So, I asked him to use the script from the following blog to find out if there are other backups running.
Sure enough, SQLDBAs were not aware of DPM and they configured native SQL Server backup. Since DPM was already protecting the server, we disabled the job which was taking T-Log backups. After this issue was resolved and same error never came back.
Reference: Pinal Dave (https://blog.sqlauthority.com)