Sql-server – Understanding CleanupTime in Ola Hallengren’s SQL Server scripts in relation to FULL backups and LOG backups

backupola-hallengrensql serversql-server-2016

I am having trouble understanding what exactly to expect from the CleanupTime option in the Ola Hallengren Server Maintenance Solution. I'm finding some related questions, and elaborate answers, but the explanations still puzzle me a bit.

Specifically:

I am doing a weekly FULL backup, a daily DIFF backup, and an hourly LOG backup.
The FULL backup is using the default CleanupTime of 24h. The DIFF and LOG backup have NULL as CleanupTime.

From the documentation of the CleanupTime paramter, I fail to understand if setting the CleanupTime setting for a backup of BackupType FULL, will also delete older DIFF and LOG backup files, or only FULL backup files.

Specify the time, in hours, after which the backup files are deleted.
If no time is specified, then no backup files are deleted.

The latter paragraph makes me think that setting CleanupTime on backups of BackupType FULL will also delete older transaction logs. Yet it is unclear if this paragraph only applies to backups of the BackupType LOG, or also to backups of the BackupType FULL.

DatabaseBackup has a check to verify that transaction log backups that
are newer than the most recent full or differential backup are not
deleted.

What I am trying to achieve, is that I can do a point-in-time recovery up to 1 week. (We have a very slowly changing database, so this is feasible) The way I understand it now, this would require a week old full backup, and a weeks worth of Transaction log backup. Since the full, and differential backups can only be used to restore to one specific point in time.

So, should I just set the CleanupTime option of my FULL backup job to 24*7? What I'm guessing now is that setting it to 24h, will cause the next FULL backup to delete all older Full, diff and Transaction log backup files, leaving me with a point-in-time recovery window of … 0 hours. Right?

Best Answer

The @CleanupTime is always specified for a specific backup job. For example, if you create a Full backup job, a Differential backup job and a Transaction Log backup job, then the @CleanupTime always relates to the extension of the job.

Let's take a look at a Full backup example.

Full Backup

If you create a full backup job, then you will normally add one or more of the following parameters:

  • @Databases : Which databases get backed up (not really relevant for this example)
  • @Directory : The directory to store the backups
  • @BackupType : Full, Differential, TLog
  • @CleanupTime : How much hours worth of backups to keep
  • @FileExtensionFull : The extension of your backup.

So you have a backup job in place that will create a full backup according to the schedule you defined for the at job. Let's assume the following:

  • this job runs at 20:00 (8 p.m.)
  • @FileExtensionFull has been set to 'BAK'
  • @Directory has been set to 'F:\SQLBACKUP'
  • @CleanupTime has been set to 24 (hours)

If we look at the MaintenanceSolution.sql file then you will find the description for the parameter:

SET @CleanupTime         = NULL         
-- Time in hours, after which backup files are deleted. If no time is specified, then no backup files are deleted.

Well, that's not helping much. Same as in the official documentation on the site. Let's dig further. If you scan through the script you will eventually find a section that looks like this:

The script has been wrapped to increase readability

IF @BackupSoftware IS NULL
    BEGIN
        SET @CurrentCommandType02 = 'xp_delete_file'
        SET @CurrentCommand02 = 
            'DECLARE @ReturnCode int 
            EXECUTE @ReturnCode = [master].dbo.xp_delete_file 0, 
                N''' + REPLACE(@CurrentDirectoryPath,'''','''''') + ''', -- first parameter
                ''' + @CurrentFileExtension + ''', --second parameter
                ''' + CONVERT(nvarchar(19),@CurrentCleanupDate,126) + ''' -- third parameter
        IF @ReturnCode  0 RAISERROR(''Error deleting files.'', 16, 1)'
    END

So Ola is basically using the built-in xp_delete_file function of SQL Server to delete a file at a certain time according to:

  • @CurrentDirectoryPath
  • @CurrentFileExtension
  • @CurrentCleanupDate

But wait what would for example, the @CurrentCleanupDate be? If we go back a bit in the script you can find a section that looks like this:

INSERT INTO @CurrentCleanupDates (CleanupDate, Mirror)
    SELECT DATEADD(hh,-(@CleanupTime),GETDATE()), 0

Ah, so the @CurrentCleanupDate is a date addition which is calculated from the @CleanupTime and the current time GETDATE(). Cool.
(...and we might have just found a typo in the code, because the sections for normal and mirror database both contain Mirror in the code.)

What is the relevant section then for @CurrentFileExtension? Let's search around a bit again. And we find:

SELECT @CurrentFileExtension = CASE
    WHEN @CurrentBackupType = 'FULL' THEN @FileExtensionFull
    WHEN @CurrentBackupType = 'DIFF' THEN @FileExtensionDiff
    WHEN @CurrentBackupType = 'LOG' THEN @FileExtensionLog
    END

So there you have it.

Summary

If the parameters for your Full backup job are set as @FileExtensionFull='BAK' and you have set a @CleanupTime=24 then the procedure will delete all Full backup files that are at least a day old (24 hours).

The @CurrentCommand02 that gets executed is basically:

xp_delete_file 0, 'F:\SQLBACKUP', 'BAK', '2018-08-20 20:00:00.045'

So it doesn't touch on any other backup files. (Unless of course you have defined 'BAK' to be the extension of all backup types, in which case you lose).

Related Question