Inside The Storage Engine: Does turning on page checksums discard any torn-page protection?

This is an important question, because enabling page checksums doesn’t suddenly make all allocated pages be protected by page checksums (it’s not until a page is read into the buffer pool, modified, and then written back to disk, that it gets a page checksum). If all the existing torn-page protection is discarded when page checksums are enabled, then the pages would be unprotected until they got page checksums on. I couldn’t remember the answer, so I experimented!

My idea was to create a database with torn-page protection, create a table with a simulated torn-page in it, then enable page checksums and see if the torn-page was still reported.

— Create the test database
USE master;
GO
CREATE DATABASE ChecksumTest;
GO
USE ChecksumTest;
GO

— Explicitly set the database to have torn-page detection
ALTER DATABASE ChecksumTest SET PAGE_VERIFY TORN_PAGE_DETECTION;
GO

— Create a test table and insert a row.
CREATE TABLE BrokenTable (c1 INT, c2 CHAR (1000));
INSERT INTO BrokenTable VALUES (1, ‘a’);
GO

— Ensure the page is written to disk and then tossed from the buffer pool
CHECKPOINT;
GO
DBCC DROPCLEANBUFFERS;
GO

Now I’m going to examine the page. There are two bits in the page header that specify whether the page is protected by torn-page detection or with a page checksum. Specifically, the m_flagBits field will have 0x100 set if the page is encoded for torn-page protection, and 0x200 set if the page has a page-checksum stored on it, and the page has not been modified (i.e. the checksum is stillvalid). You should not see the 0x100 bit set as torn-page encoding is removed when the page is read into the buffer pool – UNLESS the page IS actually torn, in which case the encoding is NOT removed.

sp_allocationmetadata ‘BrokenTable’;
GO
DBCC TRACEON (3604);
GO
DBCC PAGE (‘ChecksumTest’, 1, 143, 3);
GO

<snip>

m_pageId = (1:143)                   m_headerVersion = 1                  m_type = 1
m_typeFlagBits = 0x4                 m_level = 0                          m_flagBits = 0x8000
m_objId (AllocUnitId.idObj) = 67     m_indexId (AllocUnitId.idInd) = 256 
Metadata: AllocUnitId = 72057594042318848                                
Metadata: PartitionId = 72057594038321152                                 Metadata: IndexId = 0
Metadata: ObjectId = 2073058421      m_prevPage = (0:0)                   m_nextPage = (0:0)
pminlen = 1008                       m_slotCnt = 2                        m_freeCnt = 6070
m_freeData = 2118                    m_reservedCnt = 0                    m_lsn = (28:183:2)
m_xactReserved = 0                   m_xdesId = (0:0)                     m_ghostRecCnt = 0
m_tornBits = 770      

<snip>     

In this case the torn-page encoding has been removed, and the page is fine. Once I’ve corrupted the page on disk, it’s tricky to be able to see it with DBCC PAGE. I managed to catch it once and saw the following: