SQLSweet16!, Episode 6: DBCC CHECKDB with MAXDOP

Reviewed By: Dimitri Furman

DBCC CHECKDB is a common database maintenance task. It can take up significant amount of system resources, and can impact the performance of the production workload. There are some very good articles on the web on optimizing performance of DBCC CHECKDB and minimizing performance impact. SQL Server 2016 (and now backported to SQL Server 2014 SP2) provides another lever to manage resources consumed by DBCC CHECKDB. Now you can apply a MAXDOP option to the DBCC CHECKDB command (and to DBCC CHECKTABLE and DBCC CHECKFILEGROUP commands as well).

When MAXDOP is not specified with DBCC CHECKDB, the command uses the instance level “max degree of parallelism” configuration option. If the instance level configuration is 0 (default), DBCC CHECKDB could employ all the processors on the server and consume lots of resources, leaving very little room for the application workload. When a lower MAXDOP is used, less resources are used, but CHECKDB would take longer to finish.

The syntax of specifying MAXDOP to DBCC CHECKDB is pretty simple:

DBCC CHECKDB WITH MAXDOP = 4

Note that this command respects the MAX_DOP value that may be specified for the Resource Governor workload group used for the session running the command. If the MAXDOP value specified in the DBCC CHECKDB command is greater than the one in the Resource Governor configuration, then the latter will be used.

Figure 1 shows the elapsed time and CPU percentage for a DBCC CHECKDB test with and without MAXDOP.

Figure 1: DBCC CHECKDB with and without MAXDOP

Figure 1: DBCC CHECKDB with and without MAXDOP

In the above test, the server has default MAXDOP setting of 0. The server is 24-cores and the database size is about 190 GB. This shows that as the MAXDOP for the DBCC CHECKDB command is lowered from 0 (meaning all 24 cores) to 4, the time it takes to run increased from about 400 seconds to about 1100 seconds, while average CPU utilization is reduced from about 70% to about 10%, making the impact of DBCC CHECKDB on the application workload nearly negligible. Your mileage will vary, depending upon your hardware configuration.