Replication Extended Events

Drew Furgiuele goes hunting for the most dangerous creature of all, replication-related extended events:

Extended events are great; they have all the goodness of profiler except you don’t use profiler. Win/win! More to the point, extended events let you quickly and easily view, sort, and aggregate events that occur on your instances. They also have powerful filters (really, a “where” clause) to limit noise. You have way more control over what you monitor, how you store the data, and how you view and use it. This makes them perfect use to track replicated transactions, since we want to measure at both an individual level and the aggregate.

I fired up management studio and went to “New Session” looking for some replication event goodness and I found…

… nothing. I tried looking for events that had even parts of the name replication in it. No such thing, apparently.

This doesn’t deter Drew and he ends up building some interesting events to infer the correct answers.

Retaining system_health Metrics

Taiob Ali looks at the system_health Extended Events session:

What this article does not tell you is your individual file size is 5 MB and number of maximum rollover file is 4. Meaning you will only get 20 MB of data. Once that limit is reached your older data is lost and there is no way to recover.

Often time you have an issue with the application pointing to SQL Server and you want to diagnose the problem with system_health files. You find that files already rolled over and information you are looking for is not available. I will explain how you can solve this problem and retain system_health session files for longer period.

Read on for the solution.

Invalid Database IDs In Extended Events

Jonathan Kehayias looks at a strange scenario:

I was recently emailed with a question about tracking database usage information using Extended Events and the person wanted to know why the event session was returning invalid database_id’s as a part of what it was collecting.  The funny thing is that I’ve seen this before during demo’s when I have a very specific concurrent workload running for what I am demonstrating and I used to make sure to do this exact thing so I could explain it during Immersion Events.  The TLDR answer to this is that Extended Events isn’t returning an invalid database_id at the time that the event occurs, the database_id just isn’t valid when the data is being reviewed or consumed.  How could that be you wonder?

The explanation turns out to be pretty easy, though not necessarily intuitive.

MAXDOP-Related Extended Events

Paul Randal looks at a couple of Extended Events which help you find CXPACKET waits and degrees of parallelism:

Now I’ll set up a simple Extended Events session to track down the offending code (based on the query from here). It’s very important that you query the sys.dm_xe_map_values DMV to find the correct number to use in the query for the CXPACKET wait, as these numbers often change from release to release, and even in Service Packs. For instance, CXPACKET was 191 in SQL Server 2014 RTM, but is 190 in the 2014 build I’m using.

Be very careful about running this in production, as the event will fire for *every* wait that occurs and so will likely affect your workload throughput, even though it’ll short-circuit if the wait isn’t CXPACKET. I’ll show you a better event to use lower down.

These are good events to know.

Azure SQL Database Extended Events

Arun Sirpal compares on-prem extended events to what’s available in Azure SQL Database:

There are 22 actions and 261 events. Naturally less than your local based SQL Servers, for example on my local 2014 machine running the above query returned 50 actions and 284 events.

There are a few subtle differences and a couple not-so-subtle differences, so it’s worth digging into if you plan to spin up an Azure SQL Database database.

UDFs And Recompilations

Erik Darling shows how to diagnose a high recompilation problem:

So yeah, that function seems to get up to something once for every ID you pass in. Remember that in our STUFF… query, we grabbed the TOP 10 each time. In the XE session, each time we call the proc, the string splitting function compiles and executes code 10 times. Bummerino. That’s the life of a loop.

On SQL Server 2016 (and really, with any non-looping code), we can get around the constant compilations with a simple rewrite. In this case, I’m calling 2016’s STRING_SPLIT function instead of the MSTVF function.

Read on for more.

Querying The Histogram XE Target

Kendra Little shows how to query the histogram target for Extended Events:

I like using the histogram target because it’s relatively lightweight — you can “bucket” results by what you’re interested in. In my case, I was interested seeing the cumulative number of file_read events by file name.

But there’s one problem: the histogram target is stored in memory, not in a data file. If you want to query that data and store it off in a table, it’s not obvious how to do that.

Click through to figure out how to do that.

Quickstats Craziness

Lonny Niederstadt takes an in-depth look at the intersection of several features and discusses an issue:

Here’s the Connect Item involved – written by Michael Swart.

Trace flag 2390 can cause large compile time when dealing with filtered indexes. (Active)

Huh.  “What does that have to do with clustered columnstore indexes?” you might ask.  No, really, it’d be a personal favor if you *did* ask…

Ask and then read on.  This is the kind of post I’d send to someone wanting to learn how to troubleshoot issues.

Event Handling Items

Dave Mason looks at two Connect items related to working with extended events:

One Event Behind

I another post, I wrote that the XEvents event_stream target is regularly “one event behind”. There is an existing Connect item seeking a fix to this problem: QueryableXEventData and “Watch Live Data” one event behind. If you use the “Watch Live Data” grid for XEvents in SSMS, this is an important issue and worthy of your upvote. It’s also important if you ever want to access XEvent data programmatically with C# or PowerShell because the QueryableXEventData class uses the event_stream target and is also subject to the issue.

Read on for more details.

Reading Extended Event Data From Powershell

Dave Mason builds a Powershell script to parse Extended Events information:

Powershell takes center stage for this post. Previously, I showed how to handle a SQL Server Extended Event in C# by accessing the event_stream target. We can do the same thing in PowerShell. The code translates mostly line-for-line from C#. Check out the last post if you want the full back story. Otherwise, continue on for the script and some Posh-specific notes.

Read on for the code.


July 2017
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