Minimal logging in SQL Server can be beneficial. But when does it work?Session given during SQL Saturday Poland 2016

1. How minimal logging
can help me?
Bartosz Ratajczyk
#534 | KRAKÓW 2016

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SQL SATURDAY | #534 | KRAKÓW 2016

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4. The plan
• What is minimal logging?
• How does it work?
(and when?)
• What are the profits?
(and downsides)
• What is efficient logging?
• TRUNCATE TABLE logging?
SQL SATURDAY | #534 | KRAKÓW 2016

5. Bartosz Ratajczyk
SQL Server consultant
Database programmer
ETL programmer
Database administrator
MCSE: Data Platform, MCT, MCTS SQL Server 2008
http://bartekr.net | b.ratajczyk@gmail.com
SQL SATURDAY | #534 | KRAKÓW 2016

6. Transaction log
• Keeps track of every data modification in a
database
• Quantity of information saved in a
transaction log may depend on recovery
model
SQL SATURDAY | #534 | KRAKÓW 2016

7. Recovery models
• FULL
• allows point-in-time recovery
• requires log management
• BULK LOGGED
• point-in-time recovery not possible
• requires log management
• SIMPLE
• point-in-time recovery not possible
• takes care of log management

8. Minimal logging
„At a minimum, enough information has to be logged when
minimally logged operation is performed to allow SQL Server to
rollback a transaction that has failed.”
Kalen Delaney, SQL Server Internals 2012
„Minimal logging involves logging only the information that is
required to recover the transaction without supporting point-in-
time recovery.”
BOL: https://msdn.microsoft.com/en-us/library/ms191244.aspx
SQL SATURDAY | #534 | KRAKÓW 2016

9. Minimal logging (simplified)
• Contains only data alocation information *)
• So we save on I/O operations
BUT:
• Log does not contain all information required to
recover lost data
• Log backup will contain pages that changed
*) well, not exactly
SQL SATURDAY | #534 | KRAKÓW 2016

10. Logging types (simplified)
• Full – the operation has its own log record
• Efficient – the changed page has its own
log record
• Minimal – we log only page allocation
operations *)
*) well, not exactly
SQL SATURDAY | #534 | KRAKÓW 2016

11. Log investigation
SELECT *
FROM fn_dblog(NULL, NULL)
SELECT *
FROM fn_dblog(@StartLSN, @EndLSN)
SQL SATURDAY | #534 | KRAKÓW 2016

12. DEMO
Minimal logging

13. What can be minimally logged?
• BULK INSERT
• bcp
• SELECT INTO
• INSERT INTO .. SELECT
• CREATE INDEX
• ALTER INDEX REBUILD, DBCC DBREINDEX
• REORGANIZE always fully logged
• .WRITE
• WRITETEXT, UPDATETEXT
DEPRECATED
SQL SATURDAY | #534 | KRAKÓW 2016

14. Requirements summary
non-FULL recovery model
AND NOT replicated
AND (
(Heap AND TABLOCK)
OR (B-tree AND empty AND TABLOCK)
OR (B-tree AND empty AND TF-610)
OR (B-tree AND nonempty AND TF-610 AND
NEW key-range)
)
Source: Itzik Ben-Gan, http://sqlmag.com/t-sql/minimally-logged-inserts
SQL SATURDAY | #534 | KRAKÓW 2016

15. One more trick
If you can’t use TABLOCK then use:
EXEC sys.sp_tableoption
@TableNamePattern = N’dbo.Table’,
@OptionName = ’table lock on bulk load’,
@OptionValue = ’ON’
SQL SATURDAY | #534 | KRAKÓW 2016

16. Pros
• Faster transactions and log saving
• Smaller I/O
SQL SATURDAY | #534 | KRAKÓW 2016

17. And not pros
• Sensitive for data file errors
• Requires SIMPLE recovery model or switching
between FULL and BULK LOGGED
• So we won’t use Mirroring and Availability Groups
• But Log Shipping still can work
• May require TF 610 – sysadmin
• All data pages have to be written to disk before
transaction ends
SQL SATURDAY | #534 | KRAKÓW 2016

18. TRUNCATE TABLE
• Lightning fast data deletion
• Is it logged or not?
• Why is it so fast?
SQL SATURDAY | #534 | KRAKÓW 2016

19. DEMO
TRUNCATE TABLE

20. What to remember
• Minimal logging is just saving page
allocation information in transaction log *)
• It makes some operations quick, but is
sensitive for data file errors
• TRUNCATE is fully logged operation
*) well, not exactly
SQL SATURDAY | #534 | KRAKÓW 2016

21. Some things to read
• Itzik Ben-Gan „Minimally logged inserts: http://sqlmag.com/t-
sql/minimally-logged-inserts
• Gail Shaw, Tony Davis „Managing the Log in BULK_LOGGED
Recovery Model”
http://www.sqlservercentral.com/articles/Stairway+Series/94552/
• Paul Randall „The Myth that DROP and TRUNCATE TABLE are
Non-Logged” http://sqlperformance.com/2013/05/sql-
performance/drop-truncate-log-myth
• Remus Rusanu „How to read and interpret the SQL Server log”
http://rusanu.com/2014/03/10/how-to-read-and-interpret-the-sql-
server-log/
SQL SATURDAY | #534 | KRAKÓW 2016

22. Our Sponsors
11/29/201
6 |
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SQL SATURDAY | #534 | KRAKÓW 2016

23. Our Partners
11/29/201
6 |
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24. Bartosz Ratajczyk
#534 | KRAKÓW 2016