2. Agenda
• Transaction Log Architecture
• Log Records
• Checkpoints
• Transaction Log operations
• Recovery and Crash Recovery
• Recovery Models and Minimal Logging
3. Transaction Log Architecture
• Creating and growing the log file
• Log is always zero-initialized
• TF 3004 and 3605
• VLFs, Log blocks and LSNs
• Undocumented DBCC LOGINFO
• DBCC SQLPERF (Logspace)
4. Log Records
• What are they?
• Where they stayed?
• Are they all part of transactions?
• TVF fn_dblog (startLSN,endLSN) – TF 2537
• Compensation records (rollback) – only roll
forward
5. Conundrum
If a full backup is being taken, then you have
transaction updated 100 000 records but it is rolled
back why is the diff backup so large? Surely
nothing has changed.
6. Checkpoints
• Why they exists?
• Thresholds 20 ms and 100 ms?!
• Log records during checkpoint
• Log records are written to disk via three ways:
– When we have commit/rollbacked transaction
– When a data file page is written to disk
– When a log block hits the maximum of 60 kb size and
must be flushed to disk
7. Checkpoints (cont.)
• Types:
– Automatic
– Indirect (new in SQL 2012)
– Internal
– Manual
• TF 3502 (for which db), 3504 (write latency), 3605
• XE Events - checkpoint_begin and
checkpoint_end
8. Transaction Log operations
• VLF is either active or inactive
• DBCC OPENTRAN
• VLF is made inactive by log clearing (truncation)
• Circular nature of log
• log_reuse_wait_desc in sys.databases – why log
clearing did not succeed last time it was
attempted
9. Recovery and Crash Recovery
• Recovery brings db to a consistent point in time
• Types:
– Crash recovery after a system failure
– Recovery at the end of restore
– Recovery after a failover (CL, AG, Mirroring)
• REDO
• UNDO
10. Recovery and Crash Recovery (cont.)
• Recovery during restore
• Crash Recovery
• Where the crash recovery stops?!
• How SQL is recovering DBs? – TF 3408 single
thread
• Fast Recovery (from SQL 2005 onwards – only
ENT) – does not apply for restore
11. Recovery Models and Minimal Logging
• Minimal Logged vs Efficiently Logged
• FULL
• SIMPLE
• BULK-LOGGED (why to use it only temp)
• Minimally Logged:
– Creating, Dropping, Rebuilding indexes (reorganize is
always fully logged)
– Bulk operations – bcp, bulk insert etc.
12. Recovery Models and Minimal Logging
(cont.)
• pseudoSimple recovery model?
• Is switching between FULL and BULK-LOGGED
breaking the log backup chain?
• Deferred drop and TRUNCATE table (two ways)
