Microsoft sql server I/o basics: Chapter 2


Microsoft SQL Server 2005 I/O Error Message Changes and Additions



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Microsoft SQL Server 2005 I/O Error Message Changes and Additions


SQL Server 2005 has more error and message context information than did previous versions. This section outlines the significant I/O error message changes and additions.
Error 823

Error message 823 has been split into different error messages in order to provide improved context. Error message 823 in SQL Server 2005 represents an I/O transfer problem and error message 824 represents logical consistency problems. The 823 error message indicates a serious system error condition requiring the operating system issue to be resolved in order to correct the problem.

The message example shown here is the improved 823 error message text.

The operating system returned error <> to SQL Server during a <> at offset <
> in file <>. Additional messages in the SQL Server error log and system event log may provide more detail. This is a severe system-level 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.

SQL Server error 823, occurs when any one of the following API calls give you an operating system error.



  • ReadFile

  • WriteFile

  • ReadFileScatter

  • WriteFileGather

  • GetOverlappedResult

    For extended details on the 823 error, see Error message 823 may indicate hardware problems or system problems (http://support.microsoft.com/default.aspx?scid=kb;en-us;828339) on the Microsoft Web site.

During read operations, SQL Server 2005 may perform read retries before recording that an 823 error condition has occurred. For details, see Read Retry later in this paper.
Error 824

The 824 error indicates that a logical consistency error was detected during a read. A logical consistency error is a clear indication of actual damage and frequently indicates data corruption caused by a faulty I/O subsystem component.

The example text of the 824 message is shown here.

SQL Server detected a logical consistency-based I/O error: <>. It occurred during a <> of page <
> in database ID <> at offset <
> in file <>. 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.

Error types

The 824 message contains extended details about each specific logical error as outlined in the following table.

Note: An 824 error indicates a serious I/O subsystem stability problem and should be corrected immediately.


Error Type

Description

Checksum

The read resulted in a checksum failure. The checksum stored on the data page does not match the checksum as calculated after the read operation. Data on the page has been damaged and will require a restore to correct it.

Extended Data: “incorrect checksum (expected: ##; actual: ##)”

Contact your hardware manufacture for assistance.


Torn Page

The read resulted in a torn bits failure. The torn bits stored in the data page header do not match the torn bits stored in the individual sectors following the read operation. Data on the page has been damaged and will require a restore to correct it.

Extended Data: “torn page (expected signature: ##; actual signature: ##)”

Contact your hardware manufacture for assistance.


Short Transfer

The requested number of bytes were not read. For example, if the read request was for 8 KB but the returned data was only 4 KB, the condition is flagged as a short transfer error. This indicates that the file is damaged or the I/O subsystem has a severe problem transferring data to and from media.

Extended Data: “insufficient bytes transferred”



Bad Page Id

The page header does not contain the correct value for the expected page ID member. The expected page ID can be calculated using the following formula: (page id = physical offset in file / 8192 bytes). When the expected page is not returned, the bad page ID error is indicated.

Extended Data: “incorrect pageid (expected ##:##; actual ##:##)”

This is frequently a condition where the I/O subsystem returns the incorrect data during the read. Microsoft SQL Server Support investigations of these typically reveal that the I/O subsystem is returning data from the wrong offset in the file or the page contains all zeros. Contact your hardware manufacture for assistance.


Restore Pending

By using SQL Server 2005 Enterprise Edition, a single page restore can be performed to correct a corrupt page. If a page is damaged, it is marked as a bad page and any attempt to access it returns an 824 error. This indicates that a restore is required to correct the damaged page before it can be accessed.

Extended Data: “Database ID <>, Page <


> is marked RestorePending. This may indicate disk corruption. To recover from this state, perform a restore.”

Stale Read

For details about stale read errors, see Stale Read Protection later in this paper. The behavior is controlled with trace flag –T818.

Briefly, if a page has been recently written to disk and is still stored in the stale read hash table, the Log Sequence Number (LSN) stored in the hash table is compared to the LSN in the page header. If they do not match then the page is flagged as incorrect.

Example message: “stale page (a page read returned a log sequence number (LSN) (##:##:##) that is older than the last one that was written (##:##:##))”


Page Audit Failure

When trace flag –T806 is enabled, a DBCC audit is performed on the page to test for logical consistency problems. If the audit fails, the read is considered to have experienced an error.

Extended Data: “audit failure (a page read from disk failed to pass basic integrity checks)”

Page auditing can affect performance and should only be used in systems where data stability is in question.

Error 832

Error message 832 is returned when the in-memory checksum audit fails. For details about the in-memory checksum design, see Checksum in the Microsoft SQL Server 2005 Enhancements section in this document.

Following is an example of the text of the 832 error.

A page that should have been constant has changed (expected checksum: <>, actual checksum: <>, database <>, file <>, page <
>). This usually indicates a memory failure or other hardware or OS corruption.

The 832 message indicates a serious process stability problem, such as a scribbler, that could lead to data corruption and loss.


Error 833

SQL Server 2000 SP4 and SQL Server 2005 include stalled I/O warnings as described later in this document. The following is an example of the 833 text which is written in the SQL Server error log.

SQL Server has encountered <<##>> occurrence(s) of I/O requests taking longer than <<##>> seconds to complete on file [<>] in database [<>] (<>). The OS file handle is <>. The offset of the latest long I/O is: <


>

The 833 message indicates an I/O is hung, or is just taking a long time. This is likely an I/O subsystem problem. The information in the message can be used by Microsoft Platforms Support or your I/O subsystem vendor to trace the specific IRP and determine the root cause.

The following are a few reasons this error may be encountered.


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