In my previous blogs, I described the types of objects in TempDB and how they are managed. I hope that it provided you with a good working knowledge of TempDB. Now the next question is how do I configure the TempDB for my production workload? In this context, there are three common questions as follows:
1. Where should I create TempDB?
2. What should be the size of TempDB?
3. Single file vs multiple file?
Let us tackle each of these questions in Order.
More often than not, customer may not realize that the cause of the slowdown in the workload is because the TempDB is on the slower device. This can happen as workload changes over time or the TempDB was configured on a slower device. The performance of your TempDB is critical to your workload as, I had indicated in my earlier blogs, the SQL Server uses TempDB to store intermediate results as part of executing a query, for example to create a hash table or to sort as a result of order by. So what should you do? Well, it is not any different than what you will need to for your user databases. You need to measure the IO bandwidth needed to meet the demands of your workload. Since the persistence requirements of TempDB are different (i.e. no REDO needed), you may want to consider creating TempDB on its own spindles. In fact, you may even consider using RAM Disk to achieve better performance. Please refer to the KB article http://support.microsoft.com/kb/917047
Unlike user databases where you can probably estimate the growth rate of tables over time based on the workload, estimating the size of TempDB is non-trivial. The size of TempDB can be affected for example by a change in query plans, version store when there is a long running transaction and so on. The best way to estimate the size of TempDB is by running your workload in a test environment. Once you have a good estimate of the size of TempDB, use Alter Database command to set its size with a safety factor that you feel is appropriate. Never, I mean never, let the TempDB grow to its steady state size through auto-grow. You should only use auto-grow as a last resort but not as a strategy. Also, remember that the TempDB is created every time you restart a SQL Server but its size is set to either default of Model database or the size you had set using Alter Database command (the recommended option)
· Don’t rely on auto-grow to manage file sizes. You must set the appropriate size of TempDB using Alter Database command. Here are the pitfalls of auto-grow
o Auto-grow causes a pause during processing when you can least afford it
o Less of an issue with instant file initialization
o Auto-grow leads to physical fragmentation
SQL Server recommends that you create as many files as there are COREs or CPUs to minimize allocation bottlenecks. Yes, it is true that starting with SQL Server 2005, there is better caching of TempDB objects but there are cases when the caching does not work and even with caching, the size of temp objects grows beyond 1 page (# tables) or 1 extent (internal objects), the SQL Server can potentially incur the allocation bottleneck. Please refer to the allocation-bottleneck for details on what causes allocation bottleneck. Before we proceed further, you may wonder why we talk about allocation contention in the context of TempDB and why not in the context of other databases. Clearly, the allocation contention can happen even in user database. The main reason is that the objects in TempDB are created/destroyed all the time to process customer workload which leads to order of magnitude more allocation contention.
To minimize allocation bottleneck, here is the recommendation
· Spread TempDB across atleast as many equal sized files as there are COREs or CPUs. Since allocation in SQL Server is done using proportional fill, the allocation will be evenly distributed and so is the access/manipulation of the allocation structures across all files. Note, you can always have more files than COREs but you may not see much improvement. One of the questions that people often ask is if they should create multiple files on the same physical spindle or multiple? The allocation bottleneck can be reduced by creating multiple files on single or multiple spindles. Creating files across multiple spindles will help increase the IO bandwidth but will have no additional impact on allocation bottleneck.
· If you are still encountering allocation bottleneck, you may want to consider enabling TF-1118. Under this trace flag SQL Server allocates full extents to each TempDB object, thereby eliminating the contention on SGAM page. This is done at the expense of some waste of disk space in TempDB. This trace flag has been available since SQL Server 2000. With improvements in TempDB object caching in SQL Server 2005, there should be significantly less contention in allocation structures. If you see contention in SGAM pages, you may want to use this trace flag. If your workload is not encountering SGAM contention, then enabling TF-1118 will not help. Also, this TF is instance wide and impacts the user databases as well.