16/11/2011
1
Chirag Roy – Senior SQL DBA
Microsoft Certified Master: SQL Server
http://sqlking.wordpress.com
http://www.twitter.com/chiragroy
Transaction Log Architecture
Design Options for Performance
Hardware Options for Performance
Transaction Log Troubleshooting
Summary
16/11/2011
2
Physical/Logical Architecture
* http://msdn.microsoft.com/en-us/library/ms179355.aspx
VLF1 VLF2 VLF3 VLF4 VLF5
Virtual Log Files
Logical Log File
16/11/2011
3
Check T-LOG VLFs
DBCC LOGINFO
Tools to Check T-LOG -
DBCC SQLPERF(LOGSPACE)
Disk Usage Report
16/11/2011
4
Recovery Type Considerations -
Simple Recovery - Log file cleared on checkpoint
Full/Bulk Logged Recovery – Log file cleared on Log Backup
Bulk Logged Recovery
Potentially Larger Log Backups when running - • ALTER INDEX REORGANIZE
• DBCC INDEXDEFRAG
16/11/2011
5
VLF Design
Too few Large VLF’s due to poor design
Too many Small VLF’s in case of Autogrow
Smallest Log File Size can be 512KB on creation
VLF Sizing should be carefully planned according to environment needs
VLF Design
Chunk Size Number of VLFs
<= 1MB 2
>=1MB and < 64MB 4
>=64MB and < 1GB 8
1GB and larger 16
16/11/2011
6
VLF Design Recommendation
If log file designed for VLDBs > 8GB, expand Log File in Increments of 8GB on DB Creation to create 512MB VLFs
If log file designed < 8GB, size Log File as per requirements
Considerations –
Autoshrink – Switch OFF
Autogrowth by %, causes VLF Fragmentation
http://support.microsoft.com/kb/315512
VLF Fragmentation -
Leads to I/O overhead
Affects Redo/Undo phase performance
Increases database recovery/restore time
Cluster Failover Timing
16/11/2011
7
Considerations -
Place Data and Log files on separate LUNS to distribute I/O
Data Files experience Random Read/Writes
Log Files experience Sequential Read/Writes
SAN Admins need to provision LUNS optimized for the type of load
Considerations -
Change Model Database Recovery Mode to Simple
Full Recovery Database in Pseudo Simple Until First Full Backup
Runaway Log file if subsequently no Log backups are taken
Instant File Initialization does not work with Log Files
16/11/2011
8
Check T-LOG Zeroing
TRACE FLAG 3004
Considerations -
Log clearing can be affected by –
Recovery Model
Replication
Database Mirroring
Switch on Backup Compression in SQL 2008/R2
Switch ON Trace Flag 3042 to prevent Backup File pre-allocation
http://support.microsoft.com/kb/2001026
16/11/2011
9
In Large OLTP Environment Size Tempdb data and log file appropriately
Test using Autogrow
Size before going into production
Checkpoint occurs when Log File is 70% Full
Slow Disk I/O can cause delayed checkpoint
Mitigate using Alerts to notify
Manual Checkpoint precedes over System Checkpoint
TempDB - Special Case
16/11/2011
10
SCSI RAID1 SCSI RAID10 NAND based Technology
Read Speed Good Good Excellent
Write Speed Slower Good Excellent
Data Redundancy Good Good Depends!
Cost Affordable Expensive Very
Expensive
16/11/2011
11
Storage
Check the file latency within SQL Server using sys.dm_io_virtual_file_stats (db_id,file_id)
Use this script to get the latency for transaction log: select db_name(database_id),
io_stall_read_ms/num_of_reads AS ‘Avg. Read Wait(ms)',
io_stall_write_ms/num_of_writes AS ‘Avg. Write Wait(ms)'
from sys.dm_io_virtual_file_stats (DB_ID(),2)
ASYNC_IO_COMPLETION Can be for "zeroing" out a transaction log file during log creation or growth
WRITELOG Writing transaction log to disk
LOGBUFFER Indicates worker thread is waiting for a log buffer to write log blocks for a transaction
*http://blogs.msdn.com/psssql/archive/2009/11/03/the-sql-server-wait-type-repository.aspx
16/11/2011
12
sys.dm_os_waiting_tasks Wait information
Task level
Very accurate
Transient data
sys.dm_os_wait_stats Wait information
Cumulative by wait type
Persistent data
16/11/2011
13
Log_reuse_wait_desc in sys.databases
NOTHING
CHECKPOINT
LOG_BACKUP
ACTIVE_BACKUP_OR_RESTORE
ACTIVE_TRANSACTION
DATABASE_MIRRORING
REPLICATION
DATABASE_SNAPSHOT_CREATION
LOG_SCAN
OTHER_TRANSIENT
16/11/2011
14
16/11/2011
15