SQL Server – How to Reduce Transaction Log Backup Size After a Full Backup

sql serversql-server-2005

I have three maintenance plans set up to run on an Sql Server 2005 instance:

  • Weekly database optimisations followed by a full backup
  • Daily differential backup
  • Hourly transaction log backups

The hourly log backups are usually between a few hundred Kb and 10Mb depending on the level of activity, daily differentials usually grow to around 250Mb by the end of the week, and the weekly backup is about 3.5Gb.

The problem I have is that the optimisations before the full backup seem to be causing the next transaction log backup to grow to over 2x the size of the full backup, in this case 8Gb, before returning to normal.

Other than BACKUP LOG <DatabaseName> WITH TRUNCATE_ONLY, is there any way to reduce the size of that log backup, or prevent the optimisations from being recorded in the transaction log at all, as surely they will be accounted for in the full backup they precede?

Best Answer

Some interesting suggestions here, which all seem to show misunderstanding about how log backups work. A log backup contains ALL transaction log generated since the previous log backup, regardless of what full or differential backups are taken in the interim. Stopping log backups or moving to daily full backups will have no effect on the log backup sizes. The only thing that affects the transaction log is a log backup, once the log backup chain has started.

The only exception to this rule is if the log backup chain has been broken (e.g. by going to the SIMPLE recovery model, reverting from a database snapshot, truncating the log using BACKUP LOG WITH NO_LOG/TRUNCATE_ONLY), in which case the first log backup will contain all the transaction log since the last full backup - which restarts the log backup chain; or if the log backup chain hasn't been started - when you switch into FULL for the first time, you operate in a kind of pseudo-SIMPLE recovery model until the first full backup is taken.

To answer your original question, without going into the SIMPLE recovery model, you're going to have to suck up backing up all the transaction log. Depending on the actions you're taking, you could take more frequent log backups to reduce their size, or do more targeted database.

If you can post some info about the maintenance ops you're doing, I can help you optimize them. Are you, by any chance, doing index rebuilds followed by a shrink database to reclaim the space used by the index rebuilds?

If you have no other activity in the database while the maintenance is occuring, you could do the following:

  • make sure user activity is stopped
  • take a final log backup (this allows you to recover right up to the point of maintenance starting)
    • switch to the SIMPLE recovery model
    • perform maintenance - the log will truncate on each checkpoint
    • switch to the FULL recovery model and take a full backup
    • continue as normal

Hope this helps - looking forward to more info.

Thanks

[Edit: after all the discussion about whether a full backup can alter the size of a subsequent log backup (it can't) I put together a comprehensive blog post with background material and a script that proves it. Check it out at https://www.sqlskills.com/blogs/paul/misconceptions-around-the-log-and-log-backups-how-to-convince-yourself/]

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