Msg 824, Level 24, State 2, SQL Server detected a logical consistency-based I/O error: invalid protection option

Data in SQL Server is stored in 8KB pages, both in memory and on-disk. Corruption in those pages are detected using one of several different methods, configurable at the database level. The best option, and the one that should be used by default for essentially every database is “checksum protection”. Checksum protection calculates a checksum over the data stored in each page, and records that checksum in the page header. Configure the PAGE_VERIFY option to CHECKSUM via an ALTER DATABASE ... SET ... command, as shown by Microsoft here. Whenever SQL Server reads a page, it confirms the checksum stored in the page header matches the checksum calculated against the data stored on the page.

The Boy with a Bat by Walter Hawkesworth Fawkes

The Boy with a Bat by Walter Hawkesworth Fawkes – there appears to be no “invalid protection option” going on here!

The Errors

If the calculated and stored values don’t match, a Severity 24 error is raised:

Msg 824, Level 24, State 2, Line 89
SQL Server detected a logical consistency-based I/O error: incorrect checksum (expected: 0x602f8f73; actual: 0x302e8f73). It occurred during a read of page (1:328) in database ID 15 at offset 0x00000000290000 in file 'C:\data\bit_flip_test_system.mdf'.  Additional messages in the SQL Server error log or system event log may provide more detail. This is a severe error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.

You may receive the following slightly different severity-24 error message, indicating “invalid protection option”:

Msg 824, Level 24, State 2, Line 89
SQL Server detected a logical consistency-based I/O error: invalid protection option. It occurred during a read of page (1:328) in database ID 15 at offset 0x00000000290000 in file 'C:\data\bit_flip_test_system.mdf'.  Additional messages in the SQL Server error log or system event log may provide more detail. This is a severe error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.

I ran into this error on a production machine recently, and asked a question on dba.stackexchange.com to get clarification about what “invalid protection option” actually meant. Sean Gallardy provided a great answer that ended up being the catalyst for this blog post. Thanks, Sean!

Props to Mr. Randal

Paul Randal has an interesting article where he mentions the valid page-protection values for SQL Server are stored in the m_flagBits header value:

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 still valid). 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.

Meat-and-Potatoes

Since I like my blog posts to be evidence-based, lets see if we can cause the above error messages on a test database. First thing to do is create a test database. The code below will drop any database named bit_flip_test, so make sure you don’t have a database named that already!

After the database is created, we’ll set the recovery model to simple, and the PAGE_VERIFY option to CHECKSUM:

We’re backing the disk up to the NUL: device in the above example since the recovery model change only takes effect after a full backup is completed. We’re using the NUL: device in the backup statement since we don’t actually need a real backup – never do that kind of backup in a production environment!

This next bit of code will create a table inside the new database, and add a single row of data to the table:

The col column contains 8,000 bytes of the repeating value ff in hexadecimal, which makes it easy to see in the DBCC PAGE output. This helps visually separate the page header from the page data.

The following code uses DBCC PAGE to output hexadecimal page dumps of the single page used to store the single row from the dbo.bft table. DBCC TRACEON(3604) is necessary to redirect output to the client console. Normally, that output would be sent to the SQL Server Error Log.

The code above is borrowed from my post using DBCC PAGE to view row details. The DBCC PAGE command in this post is using output style 2, which includes the full page hex dump. The output from running the above code is shown below, with a bunch of repeating data omitted for brevity. The bits highlighted in red are the m_flagBits header value, which is were the PAGE_VERIFY bits live.

PAGE HEADER:

Page @0x0000020CCBE10000

m_pageId = (1:328)                  m_headerVersion = 1                 m_type = 1
m_typeFlagBits = 0x0                m_level = 0                         m_flagBits = 0x8000
m_objId (AllocUnitId.idObj) = 137   m_indexId (AllocUnitId.idInd) = 256 
Metadata: AllocUnitId = 72057594046906368                                
Metadata: PartitionId = 72057594041270272                                Metadata: IndexId = 0
Metadata: ObjectId = 565577053      m_prevPage = (0:0)                  m_nextPage = (0:0)
pminlen = 8004                      m_slotCnt = 1                       m_freeCnt = 87
m_freeData = 8103                   m_reservedCnt = 0                   m_lsn = (40:104:27)
m_xactReserved = 0                  m_xdesId = (0:0)                    m_ghostRecCnt = 0
m_tornBits = 0                      DB Frag ID = 1                      

Allocation Status

GAM (1:2) = ALLOCATED               SGAM (1:3) = NOT ALLOCATED          PFS (1:1) = 0x44 ALLOCATED 100_PCT_FULL
DIFF (1:6) = CHANGED                ML (1:7) = NOT MIN_LOGGED           

DATA:

Memory Dump @0x00000011B07FA000

00000011B07FA000:   01010000 00800001 00000000 0000441f 00000000  ..............D.....
00000011B07FA014:   00000100 89000000 5700a71f 48010000 01000000  ....‰...W.§.H.......
00000011B07FA028:   28000000 68000000 1b000000 00000000 00000000  (...h...............
00000011B07FA03C:   00000000 01000000 00000000 00000000 00000000  ....................
00000011B07FA050:   00000000 00000000 00000000 00000000 1000441f  ..................D.
00000011B07FA064:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011B07FA078:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
.
.  (many duplicate lines ommitted for brevity)
.
00000011B07FBF54:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011B07FBF68:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011B07FBF7C:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011B07FBF90:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011B07FBFA4:   01000000 00212121 21212121 21212121 21212121  .....!!!!!!!!!!!!!!!
00000011B07FBFB8:   21212121 21212121 21212121 21212121 21212121  !!!!!!!!!!!!!!!!!!!!
00000011B07FBFCC:   21212121 21212121 21212121 21212121 21212121  !!!!!!!!!!!!!!!!!!!!
00000011B07FBFE0:   21212121 21212121 21212121 21212121 21212121  !!!!!!!!!!!!!!!!!!!!
00000011B07FBFF4:   21212121 21212121 21216000                    !!!!!!!!!!`.

OFFSET TABLE:

Row - Offset                        
0 (0x0) - 96 (0x60)                 

Corrupt Away!

In the output above we see the page number is (1:328), this indicates file #1, page #328. We’ll need the file and page number to intentionally corrupt the page using the undocumented DBCC WRITEPAGE command. We’ll use that command to modify just the m_flagBits value in the header to see what effects specific changes have. In order to write to the page we need to set the database into single user mode. This allows DBCC WRITEPAGE exclusive access to Windows’ file-descriptor-block for the database file(s). That enables writes to the data without SQL Server executing its built-in error detection code. We’re effectively mimicking disk corruption with this technique, even though it’s a very specific corruption. The first parameter for DBCC WRITEPAGE is the database ID. On my system, the database ID is 15; you’ll need to modify that in the code below to suit your system. Don’t blindly run this against your system since it WILL corrupt database ID 15!

The output from the above code:

PAGE: (1:328)

BUFFER:

BUF @0x0000020CFDCC2580

bpage = 0x0000020CF2682000          bhash = 0x0000000000000000          bpageno = (1:328)
bdbid = 15                          breferences = 1                     bcputicks = 0
bsampleCount = 0                    bUse1 = 23948                       bstat = 0x809
blog = 0x15ab215a                   bnext = 0x0000000000000000          bDirtyContext = 0x0000000000000000
bstat2 = 0x0                        

PAGE HEADER:

Page @0x0000020CF2682000

m_pageId = (1:328)                  m_headerVersion = 1                 m_type = 1
m_typeFlagBits = 0x0                m_level = 0                         m_flagBits = 0x2202
m_objId (AllocUnitId.idObj) = 137   m_indexId (AllocUnitId.idInd) = 256 
Metadata: AllocUnitId = 72057594046906368                                
Metadata: PartitionId = 72057594041270272                                Metadata: IndexId = 0
Metadata: ObjectId = 565577053      m_prevPage = (0:0)                  m_nextPage = (0:0)
pminlen = 8004                      m_slotCnt = 1                       m_freeCnt = 87
m_freeData = 8103                   m_reservedCnt = 0                   m_lsn = (40:104:27)
m_xactReserved = 0                  m_xdesId = (0:0)                    m_ghostRecCnt = 0
m_tornBits = 1613729651             DB Frag ID = 1                      

Allocation Status

GAM (1:2) = ALLOCATED               SGAM (1:3) = NOT ALLOCATED          PFS (1:1) = 0x44 ALLOCATED 100_PCT_FULL
DIFF (1:6) = CHANGED                ML (1:7) = NOT MIN_LOGGED           

DATA:

Memory Dump @0x00000011B4FFA000

00000011B4FFA000:   01010000 02220001 00000000 0000441f 00000000  ....."........D.....
00000011B4FFA014:   00000100 89000000 5700a71f 48010000 01000000  ....‰...W.§.H.......
00000011B4FFA028:   28000000 68000000 1b000000 00000000 00000000  (...h...............
00000011B4FFA03C:   738f2f60 00000000 00000000 00000000 00000000  s./`................
00000011B4FFA050:   00000000 00000000 00000000 00000000 1000441f  ..................D.
00000011B4FFA064:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011B4FFA078:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011B4FFA08C:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
.
.  (many duplicate lines ommitted for brevity)
.
00000011B4FFBF68:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011B4FFBF7C:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011B4FFBF90:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011B4FFBFA4:   01000000 00212121 21212121 21212121 21212121  .....!!!!!!!!!!!!!!!
00000011B4FFBFB8:   21212121 21212121 21212121 21212121 21212121  !!!!!!!!!!!!!!!!!!!!
00000011B4FFBFCC:   21212121 21212121 21212121 21212121 21212121  !!!!!!!!!!!!!!!!!!!!
00000011B4FFBFE0:   21212121 21212121 21212121 21212121 21212121  !!!!!!!!!!!!!!!!!!!!
00000011B4FFBFF4:   21212121 21212121 21216000                    !!!!!!!!!!`.

OFFSET TABLE:

Row - Offset                        
0 (0x0) - 96 (0x60)                 

As you can see, the m_flagBits values is changed from 0x8000 to 0x2202. Attempting to read that page into the buffer pool will cause SQL Server to recognize corruption on the page, and report it with an error, as shown by this code:

Msg 824, Level 24, State 2, Line 86
SQL Server detected a logical consistency-based I/O error: incorrect checksum (expected: 0x602f8f73; actual: 0x302e8f73). It occurred during a read of page (1:328) in database ID 15 at offset 0x00000000290000 in file 'C:\data\bit_flip_test_system.mdf'.  Additional messages in the SQL Server error log or system event log may provide more detail. This is a severe error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.

For “fun”, let’s run DBCC CHECKDB against the database:

Output looks like:

Msg 8939, Level 16, State 98, Line 92
Table error: Object ID 565577053, index ID 0, partition ID 72057594041270272, alloc unit ID 72057594046906368 (type In-row data), page (1:328). Test (IS_OFF (BUF_IOERR, pBUF->bstat)) failed. Values are 133129 and -4.
Msg 8928, Level 16, State 1, Line 92
Object ID 565577053, index ID 0, partition ID 72057594041270272, alloc unit ID 72057594046906368 (type In-row data): Page (1:328) could not be processed.  See other errors for details.
CHECKDB found 0 allocation errors and 2 consistency errors in table 'bft' (object ID 565577053).
CHECKDB found 0 allocation errors and 2 consistency errors in database 'bit_flip_test'.
repair_allow_data_loss is the minimum repair level for the errors found by DBCC CHECKDB (bit_flip_test).

For this next test, you’ll need to re-create the database using the code at the top of the post. Once the database has been recreated, let’s corrupt the m_flagBits value to 0x0083.

The DBCC PAGE output looks like:

PAGE: (1:328)

BUFFER:

BUF @0x0000020CFDCC27C0

bpage = 0x0000020CF2688000          bhash = 0x0000000000000000          bpageno = (1:328)
bdbid = 15                          breferences = 3                     bcputicks = 324
bsampleCount = 1                    bUse1 = 25440                       bstat = 0x809
blog = 0x15ab215a                   bnext = 0x0000000000000000          bDirtyContext = 0x0000000000000000
bstat2 = 0x0                        

PAGE HEADER:

Page @0x0000020CF2688000

m_pageId = (1:328)                  m_headerVersion = 1                 m_type = 1
m_typeFlagBits = 0x0                m_level = 0                         m_flagBits = 0x8300
m_objId (AllocUnitId.idObj) = 137   m_indexId (AllocUnitId.idInd) = 256 
Metadata: AllocUnitId = 72057594046906368                                
Metadata: PartitionId = 72057594041270272                                Metadata: IndexId = 0
Metadata: ObjectId = 565577053      m_prevPage = (0:0)                  m_nextPage = (0:0)
pminlen = 8004                      m_slotCnt = 1                       m_freeCnt = 87
m_freeData = 8103                   m_reservedCnt = 0                   m_lsn = (40:104:27)
m_xactReserved = 0                  m_xdesId = (0:0)                    m_ghostRecCnt = 0
m_tornBits = 1613729651             DB Frag ID = 1                      

Allocation Status

GAM (1:2) = ALLOCATED               SGAM (1:3) = NOT ALLOCATED          PFS (1:1) = 0x44 ALLOCATED 100_PCT_FULL
DIFF (1:6) = CHANGED                ML (1:7) = NOT MIN_LOGGED           

DATA:

Memory Dump @0x00000011C05FA000

00000011C05FA000:   01010000 00830001 00000000 0000441f 00000000  .....ƒ........D.....
00000011C05FA014:   00000100 89000000 5700a71f 48010000 01000000  ....‰...W.§.H.......
00000011C05FA028:   28000000 68000000 1b000000 00000000 00000000  (...h...............
00000011C05FA03C:   738f2f60 00000000 00000000 00000000 00000000  s./`................
00000011C05FA050:   00000000 00000000 00000000 00000000 1000441f  ..................D.
00000011C05FA064:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011C05FA078:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011C05FA08C:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
.
.  (many duplicate lines ommitted for brevity)
.
00000011C05FBF68:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011C05FBF7C:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011C05FBF90:   ffffffff ffffffff ffffffff ffffffff ffffffff  ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
00000011C05FBFA4:   01000000 00212121 21212121 21212121 21212121  .....!!!!!!!!!!!!!!!
00000011C05FBFB8:   21212121 21212121 21212121 21212121 21212121  !!!!!!!!!!!!!!!!!!!!
00000011C05FBFCC:   21212121 21212121 21212121 21212121 21212121  !!!!!!!!!!!!!!!!!!!!
00000011C05FBFE0:   21212121 21212121 21212121 21212121 21212121  !!!!!!!!!!!!!!!!!!!!
00000011C05FBFF4:   21212121 21212121 21216000                    !!!!!!!!!!`.

OFFSET TABLE:

Row - Offset                        
0 (0x0) - 96 (0x60)                 

Let’s run a SELECT statement against the table:

Now, we get the “invalid protection option” error, confirming that specific values in the m_bitFlags header field do indeed cause that error:

Msg 824, Level 24, State 2, Line 86
SQL Server detected a logical consistency-based I/O error: invalid protection option. It occurred during a read of page (1:328) in database ID 15 at offset 0x00000000290000 in file 'C:\data\bit_flip_test_system.mdf'.  Additional messages in the SQL Server error log or system event log may provide more detail. This is a severe error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.

Summary

Page corruption can take many forms; the “invalid protection option” error message detailed in this post comes from corruption to a very specific part of the page, the m_flagBits field. If you get corruption reporting this error, you may be able to fix the page using DBCC WRITEPAGE by adjusting the m_flagBits field to a valid value.

When SQL Server detects corruption on a database, various details are recorded in the msdb database, in the dbo.suspect_pages table. Check out our post on suspect_pages for important details about that table, including required maintenance!

I hope you enjoyed this post, and please take a look at the rest of our SQL Server Internals posts!

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