A few weeks ago I wrote a post about log parallelism causing excessive log file sync waits. Ever since, I’m finding more and more examples how this bug affects OLTP and hybrid databases (and even some data warehouses)! For example, my current employer is a large organization that has several thousand databases (set up at different time by different teams), and according to the studies I conducted on a sample of a few dozen databases, no less than 15-20% of the total Oracle database real estate have this problem. In several cases the scale of the problem is simply scary: e.g. I found a database that spends 43% of its time on log file sync (!) on the average, reaching up to 78% (!!) during peak workloads. Based on the feedback to my posts here and on LinkedIn, the situation in other organizations is no better. It looks like “log file sync” is a very wide-spread problem, and log parallelism is one of the main causes (if not the main cause).
This is why I am happy to announce that Oracle development has created a bug and assigned it a number (19959089). The bug note doesn’t have much at this stage, but at least it does list setting “_log_parallelism_max” to 1 as a workaround, which should encourage more people to test and apply this solution. It would probably still be necessary to get the Oracle support to okay changing the underscore parameter, but a reference to this bug should make it much simpler to obtain. Hopefully, when the bug investigation is concluded, there will be a detailed official note and/or a patch available that would eliminate the need to raise an SR. I’ll post a blog about it when this happens.