SQL server query response time influenced by "outside" processes?

Question

Can the performance (response time) of a query executed in a DBMS like SQL Server be influenced by whatever it's happening on the machine on which the server runs? To be more specific, is the response time expected to increase when running a couple of Windows processes that continuously check and clean the machine, and process data received from the network?

Thanks.

Answer 1

The four key resources for any program are available memory, processors, disk space, and disk usage.

Let's investigate each of these in turn. Available memory is well-managed in SQL Server (see here). The default behavior is to start with a bunch of memory and then increase it as necessary. If your query load is not changing, then SQL Server should hit a maximum amount of memory and stop growing. It sounds like your query load is consistent over time, so memory would not be a big issue. Also, many configurations of SQL server fix the memory size to avoid interference with other processors.

Processing power. This can be a big one. SQL Server requires processing power. The processors may be used by other Windows processes. This would slow down queries, particularly those that are processing (as opposed to I/O) constrained. However, this might be mitigated on a multi-processor machine. A given SQL Server instance might be assigned a certain number of processors. The rest could be used for Windows.

Disk space. This has little impact. In general, either disk that is needed is available or it is not (and the query needing it fails). One exception is temporary disk space, the availability of which can influence query execution plans. Often, temporary space is put on its own drive to avoid needless conflict with other processes.

I/O bandwidth. SQL Server needs to communicate through the file system to disk. This can be a real performance drag, and it can occur at multiple different levels. The operating system itself could be saturated with I/O calls, slowing down the database. The network between the CPUs and the disks could be saturated, slowing down reading and writing speeds. The disk system itself can be slow because of multiple concurrent actions -- even from different servers. And this can get all the more complicated in a virtual environment.

The answer is "yes". Windows processes can affect the performance of SQL queries. My best guess is that the affect would be either in terms of eating up processors or using up disk bandwidth.

Answer 2

Yes, the database server is sharing resources with anything else that runs on the machine, so any resource intensive process could affect the database performance noticeably.

One important resource is the memory. The SQL Server will by default use up all free memory if it has any use for it. If you are running any other processes on the server, you should limit the memory use of SQL Server so that it allocates a bit less, to allow room for the other proccesses, to reduce memory swapping.