Sql-server – Database backup – differential file size question

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/]

You are describing log shipping, but you want to use 'differential' backups instead of log backups, which is the problem with your approach. With log shipping you restore the database once, then apply log backups as they are being created on the principal site, and you never ever have to redo the initial full backup restore. Just keep applying the log shipped over every few hours and you'll have your read-only copy available.