Misunderstanding of SQL Server: Misunderstanding of page checksum on 17th days

In fact, I have already explained in detail the page checksum: How to tell if the IO subsystem is causing when uptions?

Misunderstanding #17: Several misunderstandings about page checksum

It is basically wrong

17 a) Page CheckSum is automatically enabled after upgrading from SQL Server 2000 or 7.0

Actually, no. The page CHECKSUM is not automatically enabled for the DATABASE upgraded from the old instance, unless you explicitly enable it using alter database databasename SET PAGE_VERIFY CHECKSUM. The SQL Server 2005 or 2008 database page checksum is enabled by default.

17 B) page checksum can correct errors

Page checksum can only detect errors, rather than correcting one error as the CRC verification code in the network protocol.

17 c) When page checksum is enabled, the previous page checksum is added to each page of the database..

Error. In fact, after page checksum is enabled, no thread will modify the page in the foreground or background. The page checksum is added to the page only when you re-create an index or modify a page.

17 d) read the page to add the checksum to the page.

Error: only when the page is read to the memory and modified and written back to the disk can the page checksum be added to the page.

17 e) when the page Verification Mode of the database is changed from torn-page detection to page checksum, all torn-page detection information will be lost.

Error. Every page knows that its protection mode is torn-page detection, and the page checksum or no. As mentioned previously, only after a page is modified can its page verification method be modified. (Note: some pages in a database can be torn-page detection, other pages are page checksum methods ). I have an article that details this part in detail: Inside The Storage Engine: Does turning on page checksums discard any torn-page protection ?.

17 f) page checksum can be detected immediately when an error occurs

This misunderstanding was revealed by the prompt of SQL Server MVP Gail Shaw. Of course, it is also wrong. The damaged page will not be found until it is read into the buffer pool of the memory for validity verification.