Why could restoring a log-shipping log backup be slow?

A short post to start the day (in India) and then at lunchtime I'll do how it works: FILESTREAM garbage collection.

This is a question that came up recently on an MVP discussion list: why does a 200MB log backup take 40 minutes to restore on my log-shipping secondary?

First answer I thought of was that the I/O subsystem on the log-shipping secondary was very slow, or broken in some way, so I/Os were taking ages to complete. Nope – I/O subsystem was performing well.

Next thing I suggested was the the log-shipping secondary was restoring the log backups using WITH STANDBY and there was something like an index rebuild that was being rolled back. As an aside, when you restore log backups on the log-shipping secondary, you have a choice how they recovery: either WITH NORECOVERY or WITH STANDBY. The first option doesn't allow any access to the database, as the database is still "in recovery". The second option runs the REDO part of recovery, then runs the UNDO part of recovery, but saves the details of what it had to do for UNDO into a file (who's name and location you specify). It then allows read-only access to the database, for queries/reporting/whatever. Obviously if there's a lot of transaction log that has to be undone (i.e. rolling back transactions that weren't committed at the time the log backup completed), then this could take some time. But 40 minutes? No. That wasn't it.

I was partly right on my second guess. The *previous* log backup that was restored WITH STANDBY contained a long-running index operation, and so the undo file that the restore created was *huge*. The next log backup that's restored after a RESTORE … WITH STANDBY, must first undo everything in the undo file (i.e. put the database back to the exact state it was in as if the WITH STANDBY part of the restore never happened) before it can restore more transaction log. In this case, no-one had noticed that the previous log restore *also* took a lot of time and created the huge undo file. It just looked like the 200MB log backup was causing the problem.

My advice: in any kind of there's-suddenly-a-corruption-or-performance-problem situation, don't just look at what's happening now. Always look at what happened leading up to the problem, as the answer is often there.

4 thoughts on “Why could restoring a log-shipping log backup be slow?

  1. Another behaviour I seen if tlog backup contains the large expanstion to tlog file size obviously it will take time to initialize tlog file.

  2. Hi,
    in My case, There are no Maintenance jobs are running in primary server(or Primary Database). Also i didn’t notice any abnormal issues in Secondary server and there is another Database configured for log shipping as a secondary as a standby mode and the restoration for that database is working fine with out any issues and it is faster in restoration though the backup files are nearly 400 MB but the concern database backup size are 10 MB taking nearly 10 min for restoration (it’s also kept as standby mode).

    it’s a big surprise to me IO subsystem also good in Primary as well as secondary server.

  3. I started seeing extremely slow LS restores ~300 kbs/sec. The set up is tlog backups every 5 minutes, copied to secondary every 5 minutes, Restore backups once per hour, leaving the db in standby. The backups are small (~100MBs) and I’m getting restore times > 5 minutes/file, so I fall behind during the day. SLEEP_BPOOL_FLUSH is the wait type for the spid doing the restore. The last file of each restore cycle is always fast – maybe something with rolling forward/back each of the restores? (except for the last one!). Verified VLFs are ~200 on the primary, so that’s not the problem. For the scheduler handling the restore, the pending_disk_io_count can sometimes hit 200K. Will try switching to restore every 5 minutes to see if that “fixes” the problem – this would perhaps indicate a problem with going to and from Standby between each file restore? Any help out there? Thanks for any and all advice.

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