Tuesday 10 December 2013

T-SQL Tuesday #49: Wait for It …

T-SQL Tuesday #49
This is a post inspired by the T-SQL Tuesday blog party series; this month (#49) the topic is hosted by Robert Davis (@SQLSoldier). Robert challenged us to "give us your most interesting post involving waits". Which reminds me...

I used to live in a little hamlet in the north of Devon. The closest post office was about 5 miles away, and this was so far in the sticks that the post office and village shop were second only to the pub for being the centre of the conversationalist's universe. As this was both in one, you can imagine how much information flowed through that little building!

However, this wasn't all great news if you were in a hurry. Occasionally, I'd pop across to post some item that needed weighing, or proof of purchase, and discover that in order to buy my stamp and post the letter, I'd have to wait for the conversation in front of me to finish. There was only one serving position, so It wasn't unknown for a queue with two people in it to take upwards of 45 mins. There was a similar queue if you wanted to buy stationary or similar from the shop desk (it seems to be a British tradition that the post office queue is separate from the queue for the rest of the shop - even if the same person then serves you).

So, I hear you ask, where is this going? What relevance does this have? Well, the post office / corner shop metaphor is a reasonable one for some of the waits in SQL Server (and I believe a reasonable attempt at a somewhat humourous twist on the topic at hand). 

We have the queue - the "to do" list, of people waiting to be served - queries or processes which are waiting to be worked on. We have the thing they are waiting for - time at the counter - this could represent resources such as RAM, with the worker who serves us as the CPU. We have the question of whether there is more than one person serving at a time (Parallelism). Perhaps we have a post office and a corner shop (2 instances) in the same building (server).

What about the shop owner (DBA)? He has to plan for whether he has enough capacity (resources) for the people coming to the shop (workload). He needs a big enough doorway (IO) to cope with the people coming and going, with their parcels (data). He doesn't want to pay for (license) more staff than he has need for, but neither does he want people to dread coming to his shop (Server) to do their business (get, put or update the required data) because it takes too long - otherwise he may lose his contract to run the post office (job)! 

In the day to day process of managing the flow of people through a post office, our shop owner needs to ensure he has information about why people aren't happy, so that he can solve it. Our DBA has these tools however, and one of these is the wait stats view sys.dm_os_wait_stats, which gives the aggregated view of waits that the system has encountered - the equivalent of an anonymous survey of all customers using the shop during the day asking what they waited for, which gives us the tools to see where we might be stretched for resources. As with the survey, sometimes the results are a little misleading, and so we need to dig further to cure the problem, but it gives a good place to start looking.

A lot of the problems in this simple metaphor are familiar to us all, but I find that relating the technical details to a real world environment can help to clearly explain why things aren't working, and perhaps help us to solve a constraint on our work.

Lastly, yes I know that this was strictly about waits involving post, rather than a post involving waits... ;-)

1 comment:

  1. Interesting. I love analogies for explaining how things work! Thanks for joining in on T-SQL Tuesday #49.