Torn I/O is often referred to as a torn page in SQL Server documentation. A torn I/O occurs when a partial write takes place, leaving the data in an invalid state. SQL Server 2000/7.0 data pages are 8 KB in size. A torn data page for SQL Server occurs when only a portion of the 8 KB is correctly written to or retrieved from stable media.
SQL Server always checks I/O completion status for any operating system error conditions and proper data transfer size and then handles errors appropriately. Torn pages generally arise after system outages where the subsystem does not complete the entire 8-KB I/O request.
Disk drive manufacturers generally limit data transfer guarantees to sector boundaries of 512 bytes, so if the stable media is a physical disk and the controller does not contain a cache with battery backup capabilities, the I/O request is limited to the final spin/transfer rate of the physical drive. Thus, if an I/O is issued to write 8 KB (for a total of sixteen 512-byte sectors), but only the first three sectors make it to stable media, then the page becomes torn resulting in data corruption. A subsequent read of the 8-KB page would bring in 3 sectors of the new version of the page and 13 sectors of an older version.
SQL Server can enable torn page detection on a per-database basis. A portion of the first 512-byte sector of the page contains the page header. When torn page detection is enabled, the header member contains information about each of the other 512-byte sectors or segments of the 8-KB page. When the page is read in, the torn page information is used to detect a torn page situation.
Torn page detection incurs minimal overhead and is a recommended practice for SQL Server installations
Starting with SQL Server 2005 database level page verify "CHECKSUM" is the default and in SQL Server 2008 it is enabled for "tempdb" too.
However "Torn page detection " is not default for SQL Server 2000. Also databases upgraded from previous version on SQL Server 2005 and 2008 will not have "Page Verify" enabled.
References: SQL Server 2000 I/O Basics
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