Interview Question: What are Wait Stats in SQL Server?

By -

This should required only a short answer, such as “It provides statistics on what SQL Server is waiting for. For example, there is a Stat for Logging (LOGBUFFER, LOGMGR_QUEUE). One of the most important ones is the wait time for writing to media (WRITELOG).” Asking for the precise name is a bit unfair – such a question would be testing rote/memory abilities and not understand of what is going on. Some more important names are IO_COMPLETION, SOS_SCHEDULER_YIELD, PAGELATCH_XX, PAGEIOLATCH_XX, etc.

 

You could ask about general types, for example: Resource Waits (i.e. internal resources), External Waits and Queue Waits. However, a poor response may easily occur if the question is not very carefully phrased --

 

However, asking what you would do to resolve a high WRITELOG or LOGBUFFER statistics is a much better question. The typical answers could include:

  • If you are using a virtual machine, make sure that data disks are passthru and not virtual disks
  • Change the RAID configuration, add more storage Devices
  • Check the bandwidth to the storage devices
  • Isolate the Log files from the database files onto different storage devices
  • Install storage devices that are faster

A follow-up question could be “How do you get the stats?” This could be legitimately answered with “I have some TSQL code that I cut and paste..”  Some people may reference Sys.dm_os_wait_stats. For an example of such code, see Brent Ozar code example or Paul Randal code example.  As an interesting aside (unfair to ask), is that this code sample works with SQL Azure.

 

A common aspect of interviews are questions that may test irrelevant knowledge. Asking a MD to explain why mercury is used in a thermometer instead of water is really not asking about he needs to know.  Asking a family practice candidate MD to describe the difference in SPECT scans between an Alzheimer's patient and a Chronic Lyme patient is similarly irrelevant. The best questions that I have ever been asked are the ones that the interviewer are currently struggling with. I have been asked a lot of irrelevant to the job questions, way too many IMHO.

Comments

Post a Comment

Popular posts from this blog

Yet once more into the breech (of altered programming logic)

By -
Once upon a time, long long ago, I wrote 8-bit programs in Tiny-C with absolute addressing being the only reality.  Then we had Borland Turbo-C with relative addresses and the joys of relative assembly jumps. Enter Windows and the joys of locking and unlocking memory blocks that you must allocated and manage yourself. Then came Visual Basic which evolved into .Net CLR and decreasing awareness of what is happening under the hood.   With VS2010 including F# and enhanced parallelism in .Net4, we actually have to go retro a bit – back to the world of having to understand the hardware. CPU speeds are no longer increasing – I have an old laptop from six+ years ago that ran at 3.2 GHz. This year I updated one of my machines off-the-shelf from Costco.com, an I7-920 with 4 cores and hyper-threading (8 CPUs appears in Task Manager) and 12 GB of DDR3 RAM. This week I ordered from Costco a Phenom II X6 (Yes – 6 REAL CORES) for one of my daughters.  This CPU will (according to YouTube demos) o
Read more

Simple WP7 Mango App for Background Tasks, Toast, and Tiles: Code Explanation

By -
Image
This is a continuation of this post , explaining how the sample app works. Caution : This sample isn’t meant as an example of best patterns and practices. If you see something in the code you would never do, then don’t. Solution File Organization The app has three Visual Studio Projects: the app, the agent, and the shared code library between them. One of the great things about Mango is that now the built-in Visual Studio Unit test functionality is available. By separating out the meat of the work into the SampleShared project, I can also focus my unit testing there. SampleApp1 The app has a MainPage, a ViewModel, and the WMAppManifest.xml file modified to note my ExtendedTask of BackgroundServiceAgent. The Agent is extremely simple with a call to the shared library, an update to the Tile, and a call to pop up Toast. The meat of the work is in the SampleShared library. The AgentMgr manages the background agent : add, remove, find, run now. It is only called f
Read more

How to convert SVG data to a Png Image file Using InkScape

By -
Image
Introduction The project required the need to put the visual pie chart on a web page on the client and in a Pdf file created on the server. I eventually choose to use a Telerik Kendo DataViz Chart control on the client, and use a PNG file on the server for the Pdf file. This blog post will explain the process and code to convert the client-side pie chart Svg data to a Png file, and provide a download to the working code. Prerequisites You need to have a trial version of the Telerik Kendo library installed in order to make the sample code work. You also need InkScape installed. InkScape is the application that does the actual conversion. You will need to change the Web.config appSetting of “ExeDirectoryForInkscape” to represent your own installation location. Any SVG Data Will Work The pie chart was created using the Telerik Kendo DataViz chart control. While I name a specific control, as long as you have access to the SVG data, the same conversion process on the server will
Read more