Tempdb Not your average Database
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This document provides an overview of the TEMPDB database in SQL Server. It explains that TEMPDB is used to store temporary objects and internal objects used by SQL Server. It experiences high contention from sorting, snapshots, and other operations that utilize temporary work tables. The document recommends configuring multiple equal sized data files for TEMPDB, placing it on fast storage separate from user databases, and monitoring for contention on allocation pages. It describes how to identify contention on PFS, GAM, and SGAM pages and provides tips on addressing contention issues through additional files, faster storage, and avoiding spills to disk.
Tempdb Not your average Database
- 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