{"id":714,"date":"2010-04-17T14:01:00","date_gmt":"2010-04-17T14:01:00","guid":{"rendered":"\/blogs\/paul\/post\/A-SQL-Server-DBA-myth-a-day-(1730)-page-checksums.aspx"},"modified":"2017-07-21T11:41:12","modified_gmt":"2017-07-21T18:41:12","slug":"a-sql-server-dba-myth-a-day-1730-page-checksums","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/a-sql-server-dba-myth-a-day-1730-page-checksums\/","title":{"rendered":"A SQL Server DBA myth a day: (17\/30) page checksums"},"content":{"rendered":"

(Look in the Misconceptions<\/a> blog category for the rest of the month’s posts and check out the 60-page PDF with all the myths and misconceptions blog posts collected together when you join our Insider list<\/a>, plus my online Myths and Misconceptions training course<\/a> on Pluralsight.)<\/p>\n

A few people have suggested some of the myths around page checksums so today is another multi-mythbusting extravaganza! Well, I get excited at least :-)<\/p>\n

I described page checksums in depth in the blog post How to tell if the IO subsystem is causing corruptions?<\/a><\/p>\n

Myth #17:<\/strong> variety of myths around page checksums<\/em>.<\/p>\n

All of them are FALSE<\/em><\/u><\/strong><\/p>\n

17a) <\/strong>page checksums are enabled automatically when you upgrade from SQL Server 2000 or 7.0<\/em><\/p>\n

No. You must explicitly enable page checksums on upgraded databases using ALTER DATABASE blah SET PAGE_VERIFY CHECKSUM<\/em>. Databases that are created on later versions will have page checksums enabled automatically unless you change the setting in the model database – which you shouldn’t.<\/p>\n

17b)<\/strong> page checksums are error correcting<\/em><\/p>\n

No. Page checksums can detect errors in a page but are not like CRC-based checksums in network protocols that can correct single-bit errors.<\/p>\n

17c)<\/strong> enabling page checksums kicks off a background task to put a page checksum on each database page<\/em><\/p>\n

No. There is no process, background or otherwise, that can put a page checksum on each page. This is a major bummer (technical term :-) as it means you must perform index rebuilds or other size-of-data operations to actually put a page checksum on the pages. This myth goes hand-in-hand with 17d below…<\/p>\n

17d)<\/strong> simply reading the pages is enough to put a page checksum on them (e.g. with a backup or DBCC CHECKDB)<\/em><\/p>\n

No. A page checksum is only put on a page when it is read into memory, changed, and then written back out to disk.<\/p>\n

17e)<\/strong> when a database is changed from torn-page detection to page checksums, all torn-page detection is lost<\/em><\/p>\n

No. Pages know whether they are protected through torn-page detection, a page checksum, or nothing at all. As mentioned above, pages aren’t changed to a page checksum until they’re physically altered. I went into this in great detail with an example script in the blog post Inside The Storage Engine: Does turning on page checksums discard any torn-page protection?<\/a><\/p>\n

17f)<\/strong> page checksums detect corruption immediately<\/em><\/p>\n

This myth was suggested for debunking by fellow MVP Gail Shaw (twitter<\/a>|blog<\/a>) and is of course untrue. A damaged page cannot be detected until it is read into memory and the buffer pool checks the validity of the page checksum.<\/p>\n","protected":false},"excerpt":{"rendered":"

(Look in the Misconceptions blog category for the rest of the month’s posts and check out the 60-page PDF with all the myths and misconceptions blog posts collected together when you join our Insider list, plus my online Myths and Misconceptions training course on Pluralsight.) A few people have suggested some of the myths around […]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[30,61,63],"tags":[],"class_list":["post-714","post","type-post","status-publish","format-standard","hentry","category-corruption","category-misconceptions","category-page-checksums"],"yoast_head":"\nA SQL Server DBA myth a day: (17\/30) page checksums - Paul S. Randal<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.sqlskills.com\/blogs\/paul\/a-sql-server-dba-myth-a-day-1730-page-checksums\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"A SQL Server DBA myth a day: (17\/30) page checksums - Paul S. Randal\" \/>\n<meta property=\"og:description\" content=\"(Look in the Misconceptions blog category for the rest of the month’s posts and check out the 60-page PDF with all the myths and misconceptions blog posts collected together when you join our Insider list, plus my online Myths and Misconceptions training course on Pluralsight.) 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