This month the SQLskills team is presenting a series of blog posts aimed at helping Accidental/Junior DBAs ‘keep the SQL Server lights on’. It’s a little taster to let you know what we cover in our Immersion Event for The Accidental/Junior DBA, which we present several times each year. You can find all the other posts in this series at http://www.SQLskills.com/help/AccidentalDBA. Enjoy!
One of the most common mistakes that I see people make as they configure a new database server is to have an improperly configured or simply inadequate storage subsystem for their intended database workload. It is not that unusual for someone to specify the components for a new server with high-end Intel processors and lots of physical RAM, but with only eight internal, 10K magnetic drives for their entire storage subsystem. This is a recipe for disaster, unless you have a very light workload. A little analogy that I like to use in presentations is when you go to a gym and see guys who only work on their upper body and don’t do any leg work. They end up looking ridiculous and are not really very strong, just as a database server will be crippled by an inadequate storage subsystem.
It is always better (from a performance perspective) to have a larger number of smaller drives in a disk array, rather than a smaller number of larger drives in a disk array. Each disk, whether it is a conventional magnetic drive or a solid state drive, has certain performance limits in terms of sequential throughput (MB/sec) and random Input/Output Operations per Second (IOPS). Having more drives in the array will increase the performance of the array, at least until you run into the limits of some other component in the storage subsystem, such as the RAID controller, HBA, or the PCI-E slot you are using.
It is far too easy to simply focus on how much disk space you need for the various types of database files, such as SQL Server data files, log files, tempdb files, and database backup files rather than concentrating on the required performance metrics for each of these types of database files. You need to consider how much sequential read and write performance you need (in terms of MB/sec), and how much random read and write performance you need (in terms of IOPS) for your workload for each of these database file types. You also need to think about your disk redundancy requirements and your available budget.
Thinking about all of this will help you and your storage administrator decide how to properly configure your disk subsystem to get the best performance and redundancy possible with your available resources. This is far preferable to simply calculating your disk space requirements, and asking for a certain amount of disk space. I talk about this in much more detail in my Pluralsight course, SQL Server 2012: Evaluating and Sizing Hardware. Erin and Jonathan will also be covering this subject in Module 1 of their IE0: Immersion Event for the Accidental DBA course.
There is a free and easy to use benchmark tool that you can use to quickly compare the performance of different types of disks and disk arrays. CrystalDiskMark, which is available from Crystal Dew World is a fairly well-known disk subsystem benchmark. You can select the number of test runs, desired file size, desired test file type, and which logical drive you want to test. It allows you to measure:
- Sequential read and write performance in megabytes/second
- Random read and write performance for a 512K block size
- Random read and write performance for a 4K block size
- Random read and write performance for a 4K block size with a queue depth of 32
There are other disk benchmarks such as SQLIO that will do a much more thorough job of benchmarking your disk subsystem, but they are a little more difficult to work with. Using CrystalDiskMark should be a supplement to other disk benchmarking that you do. I like to do my first round of testing on each logical drive using CrystalDiskMark before I do more detailed, time-consuming testing with SQLIO. You should test all of your logical drives with these tools before you install SQL Server. Figure 1 shows what the results from CrystalDiskMark look like.
Figure 1: CrystalDiskMark Results for two 300GB 15K SAS drives in RAID 1
This shows you the performance characteristics for a two drive, RAID 1 array with 300GB, 15K rpm conventional magnetic drives. An array like this is commonly used for the system drive in a database server to get decent performance and to have some redundancy for the operating system and the SQL Server binaries (for a server that is not part of a fail-over cluster instance). As an accidental DBA, you need to think about your desired performance characteristics and your required redundancy levels as you decide how to lay out your physical and logical disk configuration. You have to think about the different types of database files, and how they will be used with your type of workload as you make these decisions. No matter how you decide to configure your disk subsystem, it is very important to test each logical drive with CrystalDiskMark and SQLIO before you install SQL Server, so that you don’t have any unpleasant surprises later!