Corruption Challenge 4 – Second Alternate Solution.
The following is a solution to Corruption Challenge #4 that uses DBCC WritePage to fix the corruption. Before proceeding, be sure to read this warning.
WARNING: DBCC WritePage is a dangerous command, that should never be used on any production database. It may invalidate your ability to get support from Microsoft on issues that arise with that database going forward. It is not my intention to encourage anyone to use DBCC WritePage ever. This is just what I used to create a corrupt database, and since creating corrupt databases is not part of the role of most DBAs, you should not use DBCC WritePage. Consider yourself warned.
DBCC WritePage is a cool, but extremely risky way to solve this weeks Database Corruption Challenge, however, do NOT EVER RUN THIS ON A PRODUCTION SYSTEM.
The Solution using DBCC WritePage
The following is the actual solution submitted by Raul Gonzalez for Week 4 of the Database Corruption Challenge.
USE master GO IF DB_ID('CorruptionChallenge4') IS NOT NULL BEGIN ALTER DATABASE CorruptionChallenge4 SET SINGLE_USER WITH ROLLBACK IMMEDIATE DROP DATABASE CorruptionChallenge4 END GO --RESTORE FILELISTONLY --FROM DISK = 'C:\Program Files\Microsoft SQL Server\MSSQL12.MSSQL2014\MSSQL\Backup\CorruptionChallenge4_Corrupt.bak' -- Create folder C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\DATA\ -- grant permission to SQL Server service account RESTORE DATABASE CorruptionChallenge4 FROM DISK = 'C:\Program Files\Microsoft SQL Server\MSSQL12.MSSQL2014\MSSQL\Backup\CorruptionChallenge4_Corrupt.bak' WITH NORECOVERY, REPLACE , MOVE 'CorruptionChallenge4' TO 'C:\Program Files\Microsoft SQL Server\MSSQL12.MSSQL2014\MSSQL\DATA\CorruptionChallenge4.mdf' , MOVE 'UserObjects' TO 'C:\Program Files\Microsoft SQL Server\MSSQL12.MSSQL2014\MSSQL\DATA\CorruptionChallenge4_UserObjects.ndf' , MOVE 'CorruptionChallenge4_log' TO 'C:\Program Files\Microsoft SQL Server\MSSQL12.MSSQL2014\MSSQL\DATA\CorruptionChallenge4_log.ldf' GO RESTORE DATABASE CorruptionChallenge4 WITH RECOVERY GO USE [CorruptionChallenge4] SELECT * FROM dbo.Customers WHERE id IN (510900 ,510901) -- alocation unit is not correct at page level -- Allocation unit for the CIX in the table. USE [CorruptionChallenge4] SELECT au.allocation_unit_id, * FROM sys.indexes AS ix INNER JOIN sys.partitions AS p ON p.object_id = ix.object_id AND p.index_id = ix.index_id INNER JOIN sys.allocation_units AS au ON au.container_id = p.hobt_id WHERE ix.object_id = OBJECT_ID('dbo.Customers') AND ix.index_id = 1 -- this bit is from Paul Randal PASS SUMMIT 2014 DEMO to modify the allocation unit DECLARE @alloc BIGINT = 72057594039828480; -- Allocation unit recorded in metadata DECLARE @index BIGINT; SELECT @index = CONVERT (BIGINT, CONVERT (FLOAT, @alloc) * (1 / POWER (2.0, 48)) -- right shift, reciprocal of left shift ); SELECT CONVERT (BIGINT, CONVERT (FLOAT, @alloc - (@index * CONVERT (BIGINT, POWER (2.0, 48)))) * (1 / POWER (2.0, 16)) -- right shift, reciprocal of left shift ) AS [m_objId], @index AS [m_indexId]; GO --m_objId m_indexId --29 256 -- These numbers are in decimal --replace with calculated values -- Substitute those values on each data/index page -- idobj at offset 24 for 4 bytes, idind at offset 6 for 2 bytes -- Use a calculator to convert the numbers to hex, and then byte-reverse them DBCC WRITEPAGE (N'CorruptionChallenge4', 3, 2150, 24, 4, 0x1D000000); DBCC WRITEPAGE (N'CorruptionChallenge4', 3, 2150, 6, 2, 0x0001); GO -- After this I can see the data for those rows USE [CorruptionChallenge4] SELECT * FROM dbo.Customers WHERE id IN (510900 ,510901) -- But we have to fix all pages... what about a loop? -- Get all pages for the index CREATE TABLE #dbccInd ( ID INT IDENTITY(1,1) , PageFID INT NULL , PagePID INT NULL , IAMFID INT NULL , IAMPID INT NULL , ObjectID INT NULL , IndexID INT NULL , PartitionNumber INT NULL , PartitionID BIGINT NULL , iam_chain_type SYSNAME NULL , Pagetype INT NULL , IndexLevel INT NULL , NextPageFID INT NULL , NextPagePID INT NULL , PrevPageFID INT NULL , PrevPagePID INT NULL ) INSERT INTO #dbccInd EXECUTE sp_executesql N'DBCC IND(''CorruptionChallenge4'', ''dbo.Customers'', 0)' DECLARE @count INT = 1 DECLARE @max INT = (SELECT MAX(ID) FROM #dbccInd) DECLARE @writepage SYSNAME DECLARE @pageNo INT WHILE @count <= @max BEGIN SET @pageNo = (SELECT PagePID FROM #dbccInd WHERE ID = @count AND PageType = 1) IF @pageNo IS NULL BEGIN SET @count += 1 CONTINUE END SET @writepage = N'DBCC WRITEPAGE (N''CorruptionChallenge4'', 3, ' + CONVERT(VARCHAR, @pageNo) + ', 24, 4, 0x1D000000)' EXECUTE sp_executesql @writepage SET @writepage = N'DBCC WRITEPAGE (N''CorruptionChallenge4'', 3, ' + CONVERT(VARCHAR, @pageNo) + ', 6, 2, 0x0001)' EXECUTE sp_executesql @writepage SET @count += 1 END SELECT * FROM dbo.Customers -- 511740 SELECT COUNT(*) FROM sys.objects WHERE is_ms_shipped = 0; -- 5
Every data page in the [dbo].[customers] table was corrupt, and corrupt in the exact same way. Raul was able to loop through them all to correct the corruption.
There is is, that is how Raul completed the Database Corruption Challenge this week. Nice work Raul.
WARNING: DBCC WritePage is a dangerous command, that should never be used on any production database. It may invalidate your ability to get support from Microsoft on issues that arise with that database going forward. It is not my intention to encourage anyone to use DBCC WritePage ever. This is just what I used to create a corrupt database, and since creating corrupt databases is not part of the role of most DBAs, you should not use DBCC WritePage. Consider yourself warned.
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