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CSS SQL Server Engineers

This is the official team Web Log for Microsoft Customer Service and Support (CSS) SQL Support. Posts are provided by the CSS SQL Escalation Services
  • Blog Post: How It Works: Behavior of a 1 Trillion Row Index Build (Gather Streams from SORT)

    I ran into this behavior working on a 1 trillion row, spatial index build but the behavior can apply to any Gather Streams operator, retaining the sort order as rows pass though it.   I was just surprised a bit by the behavior until I dug deeper to understand. The index was taking just short...
  • Blog Post: How It Works: FileStream (RsFx) Garbage Collection–Part (2)

    In a previous post I outlined the basics of File Stream, garbage collection: http://blogs.msdn.com/b/psssql/archive/2011/06/23/how-it-works-filestream-rsfx-garbage-collection.aspx This post continues the discussion, outlining specific details as to how the garbage collection progresses. A Single GC Thread...
  • Blog Post: Optimizing partition split when the partition is not empty

    Some of our Field Engineers, Kal Yella and Denzil Ribeiro ( @DenzilRibeiro ) have posted a blog that discusses how to optimize adding a partition when either the right most, or left most partition is not empty..  It is well worth the read, so we are posting it on PSSSQL to help get it out there...
  • Blog Post: DReplay Message: “Active connections exceed 8192, connection 8409 is waiting.”

    This message was an interesting dive into the DReplay, session boundary logic that I thought I would share.  Internally DReplay maintains a progressive, session queue.  This queue is limited to 8192 entries and populated in connection replay order based on the connect/disconnect boundaries...
  • Blog Post: SQL Server–Storage Spaces/VHDx and 4K Sector Size

    This blog outlines a new twist to my previous blog outlining issues with 4K sector sizes. SQL Server - New Drives Use 4K Sector Size: http://blogs.msdn.com/b/psssql/archive/2011/01/13/sql-server-new-drives-use-4k-sector-size.aspx In the previous post I discussed that it was unsafe for the I/O subsystem...
  • Blog Post: SQLIOSim Checksum Validations

    I had a very specific question asked of me related to the SQLIOSIM.exe, checksum validation logic.  It is pretty simple logic (on purpose) but effective so here are the basics. The key is that there are multiple memory locations used to hold the data and do the comparison.   1.     ...
  • Blog Post: Under rare conditions, using IN clause can cause unexpected SQL behavior

    I want to make you aware of a latest SQL Server 2008 hotfix documented in http://support.microsoft.com/kb/2791745 .   Using large number of constants in IN clause can result in SQL Server termination unexpectedly.   When this happens, you won’t see anything in errorlog or any dumps...
  • Blog Post: AppDomain unloading messages flooding the SQL Server error log

    This blog is built directly from a customer reported issue.  As I helped investigate the source of the issue I thought it would be of interest to a broader audience – hopefully you find this interesting, as well. Allow me to provide some history of the problem before I dive into extended details...
  • Blog Post: How It Works: CMemThread and Debugging Them

    The wait type of CMemThread shows up in outputs such as sys.dm_exec_requests.  This post is intended to explain what a CMemThread is and what you might be able to do to avoid the waits.  The easiest way to describe a CMemThread is to talk about a standard Heap, HeapCreate and the options (with...
  • Blog Post: How It Works: SQL Server (NUMA Local, Foreign and Away Memory Blocks)

    Applies to: SQL 2005, 2008, 2008 R2, and SQL 2012 versions. The NODE an operating system page, physically belongs to can be acquired using the QueryVirtualMemoryEx Windows API.  SQL Server uses this API to track locality of memory allocations. This blog is a very high level view of SQL Server behavior...
  • Blog Post: How It Works: SQL Server 32 bit PAE/AWE on (SQL 2005, 2008, and 2008 R2) – Not Using As Much RAM As Expected!

    This issue was puzzling until we stepped through the code and studied it in some detail.   The report was “SQL Server won’t use the physical memory I expect it to use.” Scenario:   SQL Server 2008 R2, 32GB RAM  - SQL only using ~22GB of total memory and won’t use anymore. Let me try...
  • Blog Post: How It Works: Gotcha: *VARCHAR(MAX) caused my queries to be slower

    The scenario: Table has a NTEXT column that the customer wanted converted to NVARCHAR(MAX) Data has both small and large storage for different rows Issued ALTER TABLE … ADD COLUMN …NVarCharColumn… NVARCHAR(MAX) Issued update MyTable set NVarCharColumn = <<NTEXT DATA>> Issued ALTER TABLE ...
  • Blog Post: How Can Reference Counting Be A Leading Memory Scribbler Cause?

    The concept of the memory scribbler comes up quite a bit in support.   The term can often be over used but I ran into a specific example that commonly fools people, including support engineers.  The random nature and even the resulting behaviors are so broad that these issues often take...
  • Blog Post: T-SQL Update Takes Much Longer Than The Matching Select Statement

    I realize the title is generic and that is because the problem is generic.   On the surface it would not surprise me that an update takes longer than a select. (A little bit anyway.) There is logging, updates to index rows, triggers, replication needs, AlwaysOn needs, perhaps page splits and...
  • Blog Post: How It Works: Online Index Rebuild - Can Cause Increased Fragmentation

    SQL Server Books Online alludes to the fragmentation possibility but does not fully explain that the Online Index rebuilding may increase fragmentation when it is allowed to run with MAX DOP > 1 and ALLOW_PAGE_LOCKS = OFF directives . The process of building an online index involves maintaining the...
  • Blog Post: Strange Sch-S / Sch-M Deadlock on Machines with 16 or More Schedulers

    Since it took me several days to track down this bug, and I did learn a couple of new things along the way, I thought I would share some of my work. 16 or More CPUS When a system presents SQL Server with 16 or more CPUs, and you are using a high end SQL Server SKU, SQL Server will enable lock partitioning...
  • Blog Post: How It Works: XEL Display in SQL Server Management Studio (SSMS) Row Limit

    This is a simple issue but if you don't expect the behavior it can surprise you. The grid, used by SSMS, is limited to a maximum number of rows, that can be displayed, of 1 million.    Note:  There is no warning dialog or flashing toolbar. Shown in the figure below is the display...
  • Blog Post: How It Works: SQL Server (BCP, Database I/O, Backup/Restore, …) Reports Operating System Error (665, 1450 or 33) when writing to the file - BIG DATA

    Suresh and I have blogged about these issues before but this post will put another spin on the information, as it applies to BIG DATA . Previous Blog References http://blogs.msdn.com/b/psssql/archive/2008/07/10/sql-server-reports-operating-system-error-1450-or-1452-or-665-retries.aspx http://blogs.msdn...
  • Blog Post: How It Works: When is the FlushCache message added to SQL Server Error Log?

    FlushCache is the SQL Server routine that performs the checkpoint operation.  The following message is output to the SQL Server error log when trace flag ( 3504 ) is enabled. 2012-05-30 02:01:56.31 spid14s     FlushCache: cleaned up 216539 bufs with 154471 writes in 69071 ms...
  • Blog Post: AlwaysON - HADRON Learning Series: Worker Pool Usage for HADRON Enabled Databases

    I am on several e-mail aliases related to Always On databases (reference Availability Group, AG, HADRON) and the question of worker thread usage is a hot topic this week.  I developed some training around this during the beta so I decided to pull out the relevant details and share them with you...
  • Blog Post: How It Works: HealthCheckTimeout Interval Activities

    As I wrote my recent blog posts and did more research I found that is would be helpful to highlight the HealthCheckTimeout behavior in more detail. Always On FCI (Failover Cluster Instance) vs Non-FCI Installations Documentation The first thing that I need to point out is the subtle wording difference...
  • Blog Post: SQL Server 2012: RML, XEvent Viewer and Distributed Replay

    Bob Ward, Keith Elmore and I establish goals for every release of SQL Server.   A primary goal is always to make supporting the SQL Server easier.  Not just in the sense of Microsoft support, although it does play a factor, but more so for you (the customer). During SQL Server 2012 planning...
  • Blog Post: How It Works: Failover Cluster/Availability Group XEL Logging Frequency

    I had a great question from my post on sp_server_diagnostics ( http://blogs.msdn.com/b/psssql/archive/2012/03/08/sql-server-2012-true-black-box-recorder.aspx ).  As ' luck ' would have it, Bob Ward, was working on some training and had a similar question.  As I investigated I uncovered a few...
  • Blog Post: How It Works: XEvent Action vs Field data values.

    I have seen several traces and questions relating to the output of the XEvent so I thought I would try to explain them a bit in an effort to reduce confusion. Terms Field == Event Data Action == Action Data - The action data is gathered from the current session/request state.  Let's look at the...
  • Blog Post: SetFileIoOverlappedRange Can Lead to Unexpected Behavior for SQL Server 2008 R2 or SQL Server 2012 (Denali)

    You should be aware the Windows bug with API = SetFileIoOverlappedRange .   This is used by SQL Server (2008 R2 and SQL 2012) only when Locked Pages has been enabled and is in use by the SQL Server. SQL Server Support has NOT encountered this in a production environment.  The issue was found...
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