SlideDeck given to SALSSA LUG 5/16/12

1. It is more than just a little system database.
•Joe Hellsten
•Senior DBA @ rackspace

2. It is installed by default but what is it?
 TEMPDB is the Junk Drawer of SQL Server “quoted by many”
• Temporary Objects
• User Objects – triggers, queries, temporary tables, variables, etc
• Internal Objects – sorts, triggers, work tables, XML variables or other LOB data type variables
• Version stores – Snapshot Isolation (SI), Read Committed Snapshot Isolation (RCSI), After
triggers, Online index rebuilds
• RCSI is cheaper than SI why?
• SQL 2012 AlwaysOn Readable Secondary makes use of tempdb for statistics and enables SI,
which leverages tempdb for row versioning.
• Other objects that use TEMPDB
 Service Broker, event notification, database mail, index creations, user-
defined functions
 SELECT
 UserObjectPages =SUM(user_object_reserved_page_count),
 UserObjectsMB =SUM(user_object_reserved_page_count)/128.0,
 InternalObjectPages =SUM(internal_object_reserved_page_count),
 InternalObjectsMB =SUM(internal_object_reserved_page_count)/128.0,
 VersionStorePages =SUM(version_store_reserved_page_count),
 VersionStoreMB =SUM(version_store_reserved_page_count)/128.0,
 FreePages =SUM(unallocated_extent_page_count),
 FreeSpaceMB =SUM(unallocated_extent_page_count)/128.0
 FROM sys.dm_db_file_space_usage;

3. What makes this guy unique?
 It is recreated each time SQL Server is started
 It is modeled after the Model database
 If it has grown, it is reset to its default when recreated
 Can not have user defined file groups created
 Auto Shrink can not be used on TEMPDB (even though it is an option)
 A database snapshot can not be created on TEMPDB
 SQL 2012 can be on local disk for clusters
 THERE IS ONLY ONE TEMPDB FOR THE ENTIRE INSTANCE!

4. A good starting point for configuring TEMPDB
TEMPDB should reside on a different disk than system
and user dbs
Depending on disk IO you might need to split the data
and log file onto different disks as well
Size TEMPDB accordingly
Make sure instant file initialization is enabled (OS
Security Policy- enable volume maintenance)
Set auto grow to a fixed value, don’t use percentage
TEMPDB should be on a fast I/O subsystem
Multiple equal size data files depending on contention

5. Did I say CONTENTION?
What is contention?
Wikipedia = “competition by users of a system for the facility at the same time”
•Latch contention on allocation pages
• Extent allocations are tracked and managed in the data files by allocation pages.
• PFS – Page Free Space. One PFS page per .5 GB of data file. Tracks how
much free space each page has. Page 1 in the data file. Repeats every 8088
pages.
• GAM – Global Allocation Map. One GAM page per 4 GB of data file.
Tracks extents. Page 2 in the data file. Repeats every 511,232 pages.
• SGAM – Shared Global Allocation Map. One SGAM page per 4 GB of data
file. Tracks extents being used as shared extents. Page 3 in the data file.
Repeats every 511,232 pages

6. How to find contention in TEMPDB
•sys.dm_os_waiting_tasks
•Any tasks waiting on PageLatch or PageIOLatch
SELECT session_id AS sessionid,
wait_duration_ms AS wait_time_in_milliseconds,
resource_description AS type_of_allocation_contention
FROM sys.dm_os_waiting_tasks
WHERE wait_type LIKE ‘Page%latch_%’
AND (resource_description LIKE ‘2:%’)
This query will give you the session, wait duration and resource
description. Resource Description is <database ID>:<file ID>:<page
number>.
Formula for page type is
 GAM: Page ID = 2 or Page ID % 511232
SGAM: Page ID = 3 or (Page ID – 1) % 511232
PFS: Page ID = 1 or Page ID % 8088

7. Raise Error
If exists (SELECT session_id, wait_type, wait_duration_ms, blocking_session_id,
resource_description,
ResourceType = Case
WHEN PageID = 1 OR PageID %8088 = 0 THEN 'Is PFS Page'
WHEN PageID = 2 OR PageID % 511232 = 0 THEN 'Is GAM Page'
WHEN PageID = 3 OR (PageID - 1) % 511232 = 0 THEN 'Is SGAM Page'
END
FROM
(SELECT session_id, wait_type, wait_duration_ms, blocking_session_id, resource_description,
CAST(RIGHT(resource_description, LEN(resource_description)
- CHARINDEX(':', resource_description, 3)) AS INT) AS PageID
FROM sys.dm_os_waiting_tasks
WHERE wait_type LIKE 'PAGE%LATCH_%'
AND resource_description LIKE '2:%'
) AS tab)
DECLARE @StringVariable NVARCHAR(50)
Set @StringVariable = (Select
ResourceType = Case
WHEN PageID = 1 OR PageID %8088 = 0 THEN 'Contention On PFS Page'
WHEN PageID = 2 OR PageID % 511232 = 0 THEN 'Contention On GAM Page'
WHEN PageID = 3 OR (PageID - 1) % 511232 = 0 THEN 'Contention On SGAM Page'
-- ELSE 'Is Not PFS, GAM, or SGAM page'
END
FROM
(SELECT top 1
CAST(RIGHT(resource_description, LEN(resource_description)
- CHARINDEX(':', resource_description, 3)) AS INT) AS PageID
FROM sys.dm_os_waiting_tasks
WHERE wait_type LIKE 'PAGE%LATCH_%'
AND resource_description LIKE '2:%'
) as tab)
RAISERROR (@StringVariable, -- Message text.
10, -- Severity,
1 -- State
) with log
go 100

8. You have contention, now what?
Contention causes transactions to queue causing extended wait times.
•Add more data files
• How many? 1 per core, 1 per 2 cores, 1 per 4 cores
• Microsoft still states 1 data file per core
• In real life it depends on concurrent processes On an 8 core system
how many concurrent processes can there be?
•Don’t over do it - to many files can have negative impact
•Create equal size files. MSSQL uses proportional fill model meaning a
larger file will be used more than the others.
•Multiple data files help reduce contention since each file has its own GAM,
SGAM and PFS information.

9. • Troubleshoot TEMPDB I/O like you would any other database
• Put TEMPDB on your fastest possible drives
• Put TEMPDB on the fastest RAID you can support
• Avoid memory overflows that spill to tempdb.
• Avoid long running queries in the version store
• Pre-size Files to Avoid Auto-growth
Options for more IOPs
• Put data files and log file on different sets of disks
• Split the individual data files to different sets of disks
Need more IOPs
Visit FUSIONIO.COM. GO with SSD’s, with over 100,000 IOPs

10. Were you paying attention?
How often should you back up TEMPDB?
How many data files per core?
How often should you shrink TEMPDB?
Where should you place your TEMPDB data and log files?
What does PFS, GAM and SGAM mean?

11. Tim Radney, Senior DBA for a top 40
US Bank
President of “Columbus GA SQL Users
Group”
Robert Davis, Idera ACE, MCM, MVP