I was trying to shrink a highly fragmented log file that had already grown beyond its default size.
Firstly, I switched the database from FULL to SIMPLE recovery mode, then backed to full for log truncation. This worked fine and the logs were truncated successfully.
After that, I did not shrink my log file immediately. Instead, I took a differential backup for bridging the recovery chain.
After that, when I was trying to shrink my log file, it seemed that the file appeared not to be shrunk, unless you take a log backup before performing the shrink.
So what is the reason behind this? The VLFs are truncated, so why can't they be shrunk?
The script below explains each step I took:
USE Master;GOIF DB_ID('ShrinkLog') IS NOT NULLBEGIN ALTER DATABASE ShrinkLog SET SINGLE_USER WITH ROLLBACK IMMEDIATE; DROP DATABASE ShrinkLog;ENDGO--Create a database for testCREATE DATABASE ShrinkLogON PRIMARY (NAME = N'ShrinkLog_Date', FILENAME = N'C:\SQLDATA\ShrinkLog_Data.Mdf', SIZE = 1024MB, MAXSIZE = UNLIMITED, FILEGROWTH = 50MB)LOG ON (NAME = N'ShrinkLog_Log', FILENAME = N'C:\SQLDATA\ShrinkLog_Log.LDF', SIZE = 1MB, MAXSIZE = 3GB, FILEGROWTH = 512kb);GO--Set recovery mode to fullALTER DATABASE ShrinkLog SET RECOVERY FULL;--Check VLFsUSE ShrinkLog;DBCC LOGINFO;--RecoveryUnitId FileId FileSize StartOffset FSeqNo Status Parity CreateLSN--0 2 253952 8192 33 2 64 0--0 2 253952 262144 0 0 0 0--0 2 253952 516096 0 0 0 0--0 2 278528 770048 0 0 0 0--Take a full backup to make sure it is in full recovery modeBACKUP DATABASE ShrinkLogTO DISK = 'NUL' WITH INIT, FORMATGO--Create a table to loadIF OBJECT_ID('dbo.Sales') IS NOT NULL DROP TABLE dbo.salesCREATE TABLE dbo.Sales( [SalesOrderID] [int] NOT NULL, [SalesOrderDetailID] [int] NOT NULL, [CarrierTrackingNumber] [nvarchar](25) NULL, [OrderQty] [smallint] NOT NULL, [ProductID] [int] NOT NULL, [SpecialOfferID] [int] NOT NULL, [UnitPrice] [money] NOT NULL, [UnitPriceDiscount] [money] NOT NULL CONSTRAINT [DF_SalesOrderDetail_UnitPriceDiscount] DEFAULT ((0.0)), [ModifiedDate] [datetime] NOT NULL,)--Load Table to grow the log file (Produce VLFs)INSERT INTO dbo.SalesSELECT TOP (350) SalesOrderID, SalesOrderDetailID, CarrierTrackingNumber, OrderQty, ProductID, SpecialOfferID, UnitPrice, UnitPriceDiscount, ModifiedDateFROM AdventureWorks2012.Sales.SalesOrderDetailGO 30--More VLFs produced and they are all active with status 2USE ShrinkLog;DBCC LOGINFO--RecoveryUnitId FileId FileSize StartOffset FSeqNo Status Parity CreateLSN--0 2 253952 8192 33 2 64 0--0 2 253952 262144 34 2 64 0--0 2 253952 516096 35 2 64 0--0 2 278528 770048 36 2 64 0--0 2 253952 1048576 37 2 64 35000000030800013--0 2 270336 1302528 38 2 64 35000000030800013--0 2 253952 1572864 39 2 64 37000000021800364--0 2 270336 1826816 40 2 64 37000000021800364--0 2 253952 2097152 41 2 64 39000000026300360--0 2 270336 2351104 0 0 0 39000000026300360--Switch to simple recovery mode to truncate logsALTER DATABASE ShrinkLog SET RECOVERY SIMPLE--Switch back to fullALTER DATABASE ShrinkLog SET RECOVERY FULL--See if log gets truncated? Yes, status becomes 0USE ShrinkLog;DBCC LOGINFO--RecoveryUnitId FileId FileSize StartOffset FSeqNo Status Parity CreateLSN--0 2 253952 8192 33 0 64 0--0 2 253952 262144 34 0 64 0--0 2 253952 516096 35 0 64 0--0 2 278528 770048 36 0 64 0--0 2 253952 1048576 37 0 64 35000000030800013--0 2 270336 1302528 38 0 64 35000000030800013--0 2 253952 1572864 39 0 64 37000000021800364--0 2 270336 1826816 40 0 64 37000000021800364--0 2 253952 2097152 41 2 64 39000000026300360--0 2 270336 2351104 0 0 0 39000000026300360--Take a diffrential backup to bridge the backup chainBACKUP DATABASE ShrinkLogTO DISK = 'NUL'WITH INIT, DIFFERENTIAL, STATS = 5GO--Shrinking does not workDBCC SHRINKFILE(2) --DbId FileId CurrentSize MinimumSize UsedPages EstimatedPages--18 2 287 128 280 128--Checking the log, VLFs can be reused but cannot be shrunk, strange...USE ShrinkLog;DBCC LOGINFO--RecoveryUnitId FileId FileSize StartOffset FSeqNo Status Parity CreateLSN--0 2 253952 8192 42 2 128 0--0 2 253952 262144 34 0 64 0--0 2 253952 516096 35 0 64 0--0 2 278528 770048 36 0 64 0--0 2 253952 1048576 37 0 64 35000000030800013--0 2 270336 1302528 38 0 64 35000000030800013--0 2 253952 1572864 39 0 64 37000000021800364--0 2 270336 1826816 40 0 64 37000000021800364--0 2 253952 2097152 41 2 64 39000000026300360--Take a log backupBACKUP LOG ShrinkLogTO DISK = 'NUL'WITH NOINIT, STATS = 5GO--Check Log, VLF 41 turns inactiveUSE ShrinkLog;DBCC LOGINFO--RecoveryUnitId FileId FileSize StartOffset FSeqNo Status Parity CreateLSN--0 2 253952 8192 42 2 128 0--0 2 253952 262144 34 0 64 0--0 2 253952 516096 35 0 64 0--0 2 278528 770048 36 0 64 0--0 2 253952 1048576 37 0 64 35000000030800013--0 2 270336 1302528 38 0 64 35000000030800013--0 2 253952 1572864 39 0 64 37000000021800364--0 2 270336 1826816 40 0 64 37000000021800364--0 2 253952 2097152 41 0 64 39000000026300360--Then you are able to shrinkDBCC SHRINKFILE(2) GO--DbId FileId CurrentSize MinimumSize UsedPages EstimatedPages--18 2 128 128 128 128
