CPU Affinity Settings on Server with Multiple SQL Server Instances


It is common to see multiple SQL Server instances being installed on one server. The intention behind this is to reduce the licensing cost related to Operating System and also to use the hardware resources effectively. Also, having multiple instances separate maintenance and provides more security from not allowing users of other applications to connect to different SQL Server instance databases. With introduction of visualization, the reason for having multiple SQL Server instances has reduced a bit by allocating on VM for each SQL Server instance thus avoiding issues with maintenance of OS or hardware. However, still there are older systems or some newer systems too which still use multiple SQL Server instances on one server.

There are some disadvantages of running multiple SQL Server instances on one server like, if built-in administrators group is added to SQL Server instance as sysadmin, then all local administrators get access to SQL Server instance by default which can be security issue, irrespective of it, a local administrator can gain access to SQL Server instance without being having permissions to the instance through backdoor by starting SQL Server instance in single user mode. Also, there can be performance issues to all SQL Server instances, if one of the SQL Server instance starts to behave bad due to bad queries, improper index and stats maintenance or bad application design.

One of the performance issue you notice on servers is high CPU usage causing performance issues. There can be one of more instances consuming more CPU thus starving other SQL Server instances, thus overall server performance degrades and application users will start to complain. One of the ways to avoid each SQL Server instance consuming more CPU and causing contention is to allocate each SQL Server instance specific CPUs available on the server. For example, if a server has 16 processors and if there are 4 SQL Server instances running on the box, we could analyse CPU requirements for each SQL Server instance based on number of databases, size of databases, number of concurrent users and max CPU required during high load, and then assign specific number of CPUs to each instance. So, for one instance if it requires more CPU usage, then allocate 8 processors to user and for remaining 3 instances use assign remaining available processors based on the usage. If all SQL Server instances usage requirement is greater than available processors, then we will need to plan to move one or more SQL Server instances on to another server.

We can use sp_configure of from instance properties, choose which CPUs to be allocated for which SQL Server instances. There is also option available to choose IO affinity mask, so do not change it, instead just change the affinity mask or CPU affinity mask. There can also be affinity 64 settings, which you can ignore and just use affinity setting or CPU affinity setting. Do not change IO affinity setting. For each SQL Server instance choose different CPUs to avoid overlapping.

Also, it is good to consider reviewing and adjusting the Max Degree of Parallelism (MaxDoP) setting as per best practices and the allocated number of CPUs for each SQL Server instance.

Hope this was helpful.

This is applicable for below versions of SQL Server

SQL Server 2005
SQL Server 2008 R2
SQL Server 2012
SQL Server 2014
SQL Server 2016

SQLServerF1 Team
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