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Category: Bugs

Indirect Checkpoint And Non-Yielding Scheduler Problems

Published 2017-06-28 by Kevin Feasel

Parikshit Savjani has a post describing an issue you might experience with indirect checkpoint post SQL Server-2012:

One of the scenarios where skewed distribution of dirty pages in the DPList is common is tempdb. Starting SQL Server 2016, indirect checkpoint is turned ON by default with target_recovery_time set to 60 for model database. Since tempdb database is derived from model during startup, it inherits this property from model database and has indirect checkpoint enabled by default. As a result of the skewed DPList distribution in tempdb, depending on the workload, you may experience excessive spinlock contention and exponential backoffs on DPList on tempdb. In scenarios when the DPList has grown very long, the recovery writer may produce a non-yielding scheduler dump as it iterates through the long list (20k-30k) and tries to acquire spinlock and waits with exponential backoff if spinlock is taken by multiple IOC routines for removal of pages.

This is worth taking a close read.

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Database Code Analysis

Published 2017-06-21 by Kevin Feasel

William Brewer has an interesting article on performing code analysis on database objects:

In general, code analysis is not just a help to the individual developer but can be useful to the entire team. This is because it makes the state and purpose of the code more visible, so that it allows everyone who is responsible for delivery to get a better idea of progress and can alert them much earlier to potential tasks and issues further down the line. It also makes everyone more aware of whatever coding standards are agreed, and what operational, security and compliance constraints there are.

Database Code analysis is a slightly more complicated topic than static code analysis as used in Agile application development. It is more complicated because you have the extra choice of dynamic code analysis to supplement static code analysis, but also because databases have several different types of code that have different conventions and considerations. There is DML (Data Manipulation Language), DDL (Data Definition Language), DCL (Data Control Language) and TCL (Transaction Control Language).  They each require rather different analysis.

William goes on to include a set of good resources, though I think database code analysis, like database testing, is a difficult job in an under-served area.

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Duplicate Key Error On DBCC CLONEDATABASE

Published 2017-06-15 by Kevin Feasel

Erin Stellato shows an edge case when you have a rather old database you’re trying to clone:

If you’ve been using DBCC CLONEDATABASE at all, you might have run into a cannot insert duplicate key error (or something similar) when trying to clone a database:

Database cloning for ‘YourDatabase’ has started with target as ‘COPY_YourDatabase’.
Msg 2601, Level 14, State 1, Line 1
Cannot insert duplicate key row in object ‘sys.sysschobjs’ with unique index ‘clst’. The duplicate key value is (1977058079).

If you do some searching, you’ll probably end up at this Connect item: DBCC DATABASECLONE fails on sys.sysowners.

The Connect item states that the problem exists because of user objects in model.  That’s not the case here.

I’m working with a database created in SQL Server 2000…now running on SQL Server 2016.

This isn’t very likely to pop up for most places (I hope!), but it’s good to know.

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Doubly Linked Lists And Bugs

Published 2017-04-06 by Kevin Feasel

Ewald Cress explains a bug in DBCC PAGE:

Let’s start with the safety convention. The “null” of a null pointer isn’t a magic value, but in real-life implementation is simply zero, which is a perfectly valid virtual address. However, on the premise that trying to access address zero or addresses near it probably indicates a program error, the OS will map that page in such a way that trying to access it causes an access violation. This is not a bug or an accident, but a damn clever feature! Robert Love explains it very nicely over here for Linux, and it applies equally to Windows.

Now recall the convention that trying to retrieve the head or tail of an empty list will – by convention – bring you back a null pointer. When iterating, a related convention may also return a zero when you’ve gone all the way around and come back to the list head. Clearly the onus is on the developer to recognise that null pointer and not dereference it, but attempting to do so sets in motion the safety feature of an access violation, which can then be neatly caught through standard exception handling, for instance yielding a diagnostic stack dump.

Very interesting article, and also a good juxtaposition of supported, “production-safe” code versus undocumented processes.

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Create Index With Drop_Existing Bug

Published 2017-04-03 by Kevin Feasel

Kendra Little describes a bug that she encountered in discussions with a reader:

My first thought was that perhaps there is some process that runs against the production system and the test system that goes to sleep with an open transaction, holding an X or an IX lock against this table. If the index create can’t get its shared lock, then it could be part of a blocking chain.

So I asked first if the index create was the head of the blocking chain, or if it was perhaps blocked by something else. The answer came back that no, the index create was NOT blocked. It was holding the shared lock for a long time.

My new friend even sent a screenshot of the index create running against the test instance in sp_WhoIsActive with blocking_session_id null.

Read on for the full story and keep those systems patched.

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Interrogating A Stack Dump

Published 2017-03-10 by Kevin Feasel

Kendra Little looks at a SQL Server stack dump:

In the video, I show an example of a stack dump caused by running DBCC PAGE with format style 3 against a table with a filtered index in SQL Server 2014.

It looks like this bug is fixed in SQL Server 2016, at least by SP1.

Sample code to reproduce this against the AdventureWorks2012 database (which I had restored to SQL Server 2014) is in my gist here.

Click through to watch the video.

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Not All Shiny Toys Are Good

Published 2017-02-15 by Kevin Feasel

Wayne Sheffield rains on our parade:

There are other issues with the MERGE statement. It has bugs… some of which can cause database corruption.

Here we have a Shiny New Toy (feature), supposed to make life easier, yet it causes problems. Until it can perform better (and the bugs are eliminated), I just don’t use it.

Beware the Shiny New Toys.

Wayne makes a great point.  Not all new things are good, even when they’re potentially quite useful.  I love shiny new toys a lot, but part of being a database administrator is protecting data, and part of that means being able to trust your tools.  Sometimes the tools work really well right out of the gate, and sometimes (like in the case of MERGE) they don’t.

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How To Create A Connect Entry

Published 2017-01-25 by Kevin Feasel

Kenneth Fisher shows how to create a Microsoft Connect entry:

I recently wrote a blog about how to tell why your SQL login isn’t working. There were a lot of good comments and several of them suggested that I create a connect entry to make the error easier to understand. There was also a question of how to create a connect entry, and it wasn’t the first time I’d heard that, this week. So I’m going to give a quick demo on how to create a connect entry. I’m not going to create it using that particular login error because, as another person pointed out, this error is actually by design. We don’t want to make it easier for someone trying to hack in right?

There are also feedback forums for Power BI and Azure.

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