SQL Server Storage Engine : TechEd 2006

TechEd interview video

Paul Randal - MSFT — Sat, 17 Feb 2007 00:25:00 GMT

Here's a video of Adam Machanic interviewing me at TechEd in Boston last year - thanks to Chuck Boyce for filming and editing.

(Hey - only 12 responses so far to the CHECKDB questions I asked in this post - I've got plenty of Hands-On labs DVDs left. So far some of the responses have let me help people with disaster recovery strategies, bug fixes and setting up con-calls with their companies for a more in-depth drill-down - maybe I could help you with something too?)

When can allocation order scans be used?

Paul Randal - MSFT — Thu, 09 Nov 2006 05:31:00 GMT

I know this is jumping the gun a little as I haven't made it this far in my series on fragmentation, but this came up in a chalk-talk I did yesterday at TechEd Developers in Barcelona and is worth blogging about.

You'd expect a select * from mytable query on a table with a clustered index to use an allocation order scan to return the data, as that's the fastest way to read all the pages at the leaf-level of an index. In fact, the query plan will show an unordered clustered index scan. Well, guess again. What I didn't remember, and thanks to SQL Server MVP Maciej Pilecki for pointing this out during my talk (even though I didn't believe him at first), is that an allocation order scan can't be used when there's any possibility of the data changing beneath the scan.

Consider an example where an allocation order scan is progressing and a page that the scan has already read then splits - adding a newly allocated page at the end of the allocation-order list of pages. The scan will eventually come to the new page and then re-read some of the rows it has already read - producing duplicates in the scan output - clearly undesirable.

The only time such a scan will be used is when there's no possibility of the data changing (e.g. when the TABLOCK hint is specified, or when the table is in a read-only database) or when its explicitly stated that we don't care (e.g. when the NOLOCK hint is specifed or under READ UNCOMMITTED isolation level). As a further twist, there's a trade-off with setup cost of the allocation order scan against the number of pages that will b read - an allocation order scan will only be used if there's more than 64 pages to be read.

Below I've included a simple script that demonstrates the behavior. For me, the funny thing is that I should have remembered this behavior as it's exactly the same reason why DBCC SHOWCONTIG takes a shared table lock when in the default mode - it's using an allocation order scan and needs to ensure that the data doesn't change and produce duplicates. Oh well - you (re)learn something every day!

-- Drop and recreate the test database

USE master;

DROP DATABASE allocationordertest;

CREATE DATABASE allocationordertest;

USE allocationordertest;

-- Create a simple table that we can fill up quickly, plus a clustered index

CREATE TABLE t1 (c1 INT, c2 VARCHAR (8000));

CREATE CLUSTERED INDEX t1c1 ON t1 (c1);

-- Add some rows, making sure to produce an out-of-order dataset when scanned in allocation order.

DECLARE @a INT;

SELECT @a = 10;

WHILE (@a < 100)

BEGIN

INSERT INTO t1 VALUES (@a, replicate ('a', 5000))

SELECT @a = @a + 1

END;

SELECT @a = 1;

WHILE (@a < 10)

BEGIN

INSERT INTO t1 VALUES (@a, replicate ('a', 5000))

SELECT @a = @a + 1

END;

-- Now try to do an allocation order scan - doesn't work...

SELECT * FROM t1;

-- Until we add the right conditions.

SELECT * FROM t1 WITH (TABLOCK);

Final TechEds of the year coming up in November

Paul Randal - MSFT — Thu, 19 Oct 2006 22:17:00 GMT

It's been a little hectic since I came back from the China/Hong Kong trip. I've been spending a lot of time getting decks ready for more TechEds and an internal training program. I've also been doing a course to get my PADI Open Water Diver certification in preparation for a two-week dive trip to Wakatobi in Indonesia I'm doing at Christmas with a friend. Phew - still need to find time for blog posts though...

The final two TechEds of the year are Tech·Ed:Developers (Nov 7-10) and Tech·Ed:IT Forum (Nov 13-17) in Barcelona. Both Don Vilen and I will be going to both and we'll be doing the following sessions:

Don

Upgrade Benefits of SQL Server 2005 for Developers
Security in SQL Server 2005
Be More Productive with SQL Server 2005 tools
SQL Querying Tips and Techniques
Performance Tuning

Paul

DBCC CHECKDB: Magic, Monsters and Myths
Database Maintenance in SQL Server 2005
End-to-End: How a query executes (once each week)

As usual, we'll be around all through the conferences and would love to spend time talking to you - stop by and say hi.

See you there!

More TechEds coming up in September...

Paul Randal - MSFT — Wed, 30 Aug 2006 23:14:00 GMT

It's been a few weeks since I last posted - I've been off on vacation around Banff, Canada plus preparing for the TechEds in China and Hong Kong.

I'll be at the following TechEds in September:

9/14 - 9/16: Shanghai

9/18 - 9/19: Guangzhou

9/21 - 9/23: Beijing

9/25 - 9/27: Kong Kong

I'll be doing the following sessions:

Title	Written By	Shanghai	Guangzhou	Beijing	Hong Kong
SQL Server Always On Technologies: Choosing the Right High Availability Solution	Matt Hollingsworth, Microsoft Michael Raheem, Microsoft	Y	Y	Y	Y
SQL Server Always On Technologies: Database Mirroring Best Practices and Performance Considerations	Sanjay Mishra, Microsoft		Y	Y	Y
SQL Server Always On Technologies: Best Practices in Building Robust, Recoverable and Reliable Systems	Kimberly L. Tripp, SQLskills.com	Y		Y	Y
SQL Server Always On Technologies: Disaster Recovery Strategies for Isolated Damage and Human Error	Kimberly L. Tripp, SQLskills.com	Y		Y	Y

And the rest of the time I'll be hanging out in the DAT cabana talking to customers. I'll be happy to do ad-hoc run-throughs of these decks and my DBCC internals and index fragmentation decks too.

If you're going to be at any of these conferences, stop by to say hi - I'd love to see you!

The best part of TechEd was meeting people...

Paul Randal - MSFT — Sat, 17 Jun 2006 06:52:00 GMT

TechEd's done and dusted for another year, and what a blast we had!

It's incredible to think of the number of customers who stopped by the SQL Server DAT section of the Technical Learning Center to pepper us with questions, and even more so to think of the number of customers who came back day after day with follow-up questions. Every evening I had a bunch of emails from customers who I'd asked to ping me for ppt decks, whitepapers, or questions - it's very gratifying to see the wide interest in the product we pour so much passion into.

My hat goes off to Michael Raheem in SQL Marketing for setting up a great selection of chalk/talks and breakouts, and based on your questions and feedback we have some really cool ideas for sessions and mini-tracks for next year.

Apart from the customers, we were lucky enough to have a bunch of SQL MVPs helping to staff the TLC. Among the people I was really pleased to meet up with again or for the first time were:

Kimberly Tripp
Andrew Kelly
Allan Hirt
Fernando Guerrero
Kevin Kline
Sean McCown
Victor Isakov

We're at TechEd - stop by and see us!

Paul Randal - MSFT — Mon, 12 Jun 2006 13:35:00 GMT

Luckily the 8.40am Alaska Airlines direct flight from Seattle>Boston was on time as I'm guessing it was 85% Microsoft!

TechEd got off to a great start yesterday with the keynotes - pretty amazing to be in a conference room with 10000 other people. For the rest of the week it'll be a whirlwind of breakout sessions, chalk/talks and Q&A. Stop by the DAT track part of the Technical Learning Centre and chat with us - just ask at the DAT info desk for any of us by name.

Look forward to seeing you!

Are you coming to TechEd? We'd like to see you there!

Paul Randal - MSFT — Tue, 06 Jun 2006 02:28:00 GMT

Just over half the Storage Engine PM team will be at TechEd June 11-17. Attending will be:

Sunil Agarwal - Access Methods PM
- specializing in locking, concurrency, bulk load, snapshot isolation, performance troubleshooting
Kevin Farlee - Transaction Services PM
- specializing in backup/restore, storage solutions, Scalable Shared Databases
Mark Wistrom - Transaction Services PM
- specializing in Database Mirroring
Don Vilen - Storage Engine Customer Communications PM
- specializing in HA, clustering, storage solutions
Paul Randal - Storage Engine Lead PM
- specializing in general DBCC, disaster recovery, CHECKDB/repair, index fragmentation

We'll all be doing TLC and/or breakout sessions and will be available for discussions all week. If you're going and would like to arrange a 1-1 on a specific topic, let me know and I'll set it up.

See you there!