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Azure PDW What is Active

Lately I have had the opportunity to work with performance tuning of queries running on the Azure Parallel Data Warehouse (Azure PDW). This has been interesting in that everything you thought you knew about SQL Server DMV’s, writing queries and overall performance tuning is just a little bit different.

My goal was to write a query to show me what is currently active and running on the Azure PDW database.

To start with, I discovered the view called SYS.DM_PDW_EXEC_REQUESTS which contains all kinds of great information to get us started. Specifically it hold information on queries that are currently running or have recently been run or attempted to be run on the Azure PDW database.

SELECT *
FROM SYS.DM_PDW_EXEC_REQUESTS;

Which returned hundreds of rows, and didn’t really get me to where I wanted to go.
Next I added a WHERE statement to filter out those queries that were “done”. In this case done means that were ‘Completed’, ‘Failed’ or ‘Cancelled’.

Read More »Azure PDW What is Active

Instant File Initialization (IFI)

One way to improve performance on SQL Server is with IFI or Instant File Initialization.

Normally files are zeroed out on a database growth during an AUTOGROW, RESTORE, CREATE DATABASE or ALTER DATABASE. This is done by SQL Server when the file grows, it runs through that file and writes zeroes to the entire new allocation in the file. The zeroing process can take a great deal of time, the Instant file initialization process skips this zeroing, and just allocates the file. This works since SQL Server will just write each 8k page to disk as they are used, thus overwriting the uninitialized file.

Running some tests on a local virtual machine running SQL Server 2012 (similar results tested on SQL Server 2014 and SQL Server 2016), here is what I found.

  • Creating a new database (10GB file size) took 2 minutes and 6 seconds, with IFI enabled, it took just 4 seconds.
  • Autogrow of a data file witha 1GB growth size took 13 seconds without IFI, with IFI it took less than a second.

 

For IFI to work, the user account that SQL Server is running as needs the “Perform volume maintenance tasks” policy to be enabled.Read More »Instant File Initialization (IFI)

Optimize for Ad Hoc Workloads

The Optimize for Ad Hoc Workloads server configuration can improve performance, and is extremely unlikely to negatively impact performance.
This was a new feature that was introduced in SQL Server 2008, and as with many new features in SQL Server, it is not turned on by default.

What Optimize for Ad Hoc Workloads Changes.

Without the Optimize for Ad Hoc Workloads feature enabled, when a plan is compiled, it is kept around in memory (cached) until something else pushes it out of the cache.

With the Optimize for Ad Hoc Workloads feature enabled, the first time a plan is compiled, it is not kept around in cache, rather a stub is kept around to indicate that they query has previously been complied. The second time the plan is compiled it is kept around in memory. Over time you end up with fewer one time use queries in memory, reducing the memory impact of the plan cache. All other multi-use ad hoc plans will need to be compiled twice on the first and second time that they are used.

What it doesn’t to

The Optimize for Ad Hoc Workloads server configuration doesn’t change the size of the plan cache in SQL Server, it does however prevent the single use ad hoc plans from using up plan cache space, making more space for other cached plans.

Read More »Optimize for Ad Hoc Workloads

mssqlsystemresource Database

I was looking through my SQL Server error logs to confirm that CheckDB was being run as I had scheduled based on my previous post to run DBCC CheckDB on all databases. I wanted to confirm that there was no corruption, and that all of the databases had been checked, and not had been missed. Going through this I noticed the logging of all of my databases, but one extra database showed up, the mssqlsystemresource database.

mssqlsystemresource database

Now the mssqlsystemresource database is an internal SQL Server database that is used by SQL Server, it also gets replaced by SQL Server when you do an upgrade of your SQL Server database. It is hidden so that people don’t have access to it, and it doesn’t show up when you run the undocumented sp_msforeachdb it doesn’t include the mssqlsystemresource database. Additionally the sys.databases view doesn’t include the mssqlsystemresource database.

Read More »mssqlsystemresource Database

Status of DBCC CheckDB

So you are checking your database with DBCC CheckDB and of course if you are like me you use the WITH NO_INFOMSGS parameter. But it turns out that CheckDB is taking longer to run that you expected, and you want to check and see what table is currently being checked.

Here is a quick SQL Script that I use to find the current table that is being checked by DBCC CheckDB

use [your database];
go

-- use this to find the session id of DBCC CheckDB
SELECT session_id, start_time, command, percent_complete, total_elapsed_time,
       estimated_completion_time, database_id, user_id, last_wait_type
  FROM sys.dm_exec_requests
 CROSS APPLY sys.dm_exec_sql_text(sql_handle)
 WHERE command like '%DBCC%';  

DECLARE @sessionID as integer = 53; -- enter the session ID of DBCC CheckDB Here

SELECT name, schema_id, type_desc
  FROM sys.objects
 WHERE object_id = (SELECT TOP 1 resource_associated_entity_id
                      FROM sys.dm_tran_locks
					 WHERE request_session_id = @sessionID
					   AND resource_type = 'OBJECT'
					   AND resource_associated_entity_id <> 50);



status of dbcc checkdb


Read More »Status of DBCC CheckDB