Log Shipping Configuration - Things to watch out

Recently I came across a situation where a server was configured to log ship. Everything was setup properly and the primary server had security rights to copy transaction log files to the secondary server.

Once configured, log shipping would work for a day and restore the logs to the secondary database as you would expect, but then it will start failing. Based on the error message, it was clear that log shipping chain was being broken somehow.

The person in charge of the database management had disabled differential backups erroneously thinking they were somehow breaking the log chain.

Remember that full backups and differential backups do not break transaction log chain. The only way you can break the log chain is either by switching the database to simple recovery model (which will truncate the log and break the chain) or if you take a backup outside of the log shipping backup process.

Log shipping consists of the following main operations:

• Backup Transaction Log
• Copy it to the secondary destination server (You may configure so that the backup file is created on the secondary server and this step won't be necessary).
• Restore the Transaction Log to the secondary database (on the secondary database).

Log shipping simply creates a transaction log backup from the last log based on LSN and restore must happen in that order, i.e. previous log must be restored first and so on. Naturally, if an outside process takes a transaction log backup, it will break the log chain and any log restores without this backup will fail. The only way to recover would be to find the missing log backup or start all over.

Indeed that was the case here. Even though there wasn't another transaction log backup process, there was a shrink log file job configured to run nightly and as part of this process, the job was first creating a backup and then shrinking the log file.

Transaction log backups will automatically truncate the log file (although it may not happen right away) but file generally won't shrink, so if you are low on disk space it is good practice to shrink the log file periodically, but you shouldn't be taking the backup, if you already have transaction log backup or log shipping configured elsewhere.

To know more about transaction log shipping and how to configure it, check out this post.

Thank you.