{"id":700,"date":"2010-04-27T15:23:00","date_gmt":"2010-04-27T15:23:00","guid":{"rendered":"\/blogs\/paul\/post\/A-SQL-Server-DBA-myth-a-day-(2730)-use-BACKUP-WITH-CHECKSUM-to-replace-DBCC-CHECKDB.aspx"},"modified":"2017-07-28T10:04:51","modified_gmt":"2017-07-28T17:04:51","slug":"a-sql-server-dba-myth-a-day-2730-use-backup-with-checksum-to-replace-dbcc-checkdb","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/a-sql-server-dba-myth-a-day-2730-use-backup-with-checksum-to-replace-dbcc-checkdb\/","title":{"rendered":"A SQL Server DBA myth a day: (27\/30) use BACKUP WITH CHECKSUM to replace DBCC CHECKDB"},"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 short one today as I’m up to my eyeballs with client work and sorting through SQL Connections abstract submissions!<\/p>\n

Myth #27:<\/strong> you can avoid having to run DBCC CHECKDB (or equivalent) by using BACKUP … WITH CHECKSUM<\/em><\/p>\n

FALSE<\/em><\/u><\/strong><\/p>\n

On first glance, this seems like it should be true as the WITH CHECKSUM<\/em> option will cause all existing page checksums on allocated pages to be checked. But it’s not and here’s why:<\/p>\n

Firstly, on a database that’s been upgraded from SQL Server 2000 or earlier, page checksums must be enabled. After they’re enabled, not all pages in the database will have page checksums on them – and an I\/O subsystem does not distinguish between pages with and without page checksums when causing corruption. So if all you do is use BACKUP … WITH CHECKSUM<\/em>, there may be corruption that you won’t find until it’s too late… (I’ll just leave the ‘until it’s too late’ hanging there… you can imagine scary consequences for yourselves :-)<\/p>\n

Secondly, you might only take a full database backup once per month, which isn’t a frequent enough consistency check for my liking (I recommend at least once per week). Even with a weekly differential backup, that’s not going to check all allocated pages in the database – only those in extents\u00a0that it backs up (those extents that changed since the last full backup).<\/p>\n

Lastly, and most insidiously, relying solely on BACKUP … WITH CHECKSUM<\/em> leaves you susceptible to in-memory corruptions. If a bad memory chip, badly-written XP, or other rogue Windows process corrupts a SQL Server data file page in memory, and then it gets written to disk, you’ve got a corrupt page with a valid checksum – and nothing will catch that except DBCC CHECKDB<\/em>.<\/p>\n

Bottom line – you can’t avoid running consistency checks. If you’re having trouble, take a look at my old blog post CHECKDB From Every Angle: Consistency Checking Options for a VLDB<\/a><\/em>.<\/p>\n

And while we’re on the subject, check out this post: Search Engine Q&A #26: Myths around causing corruption<\/a><\/em>.<\/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 short one today as I’m up to my eyeballs […]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[13,22,30,34,61],"tags":[],"class_list":["post-700","post","type-post","status-publish","format-standard","hentry","category-backuprestore","category-checkdb-from-every-angle","category-corruption","category-dbcc","category-misconceptions"],"yoast_head":"\nA SQL Server DBA myth a day: (27\/30) use BACKUP WITH CHECKSUM to replace DBCC CHECKDB - 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-2730-use-backup-with-checksum-to-replace-dbcc-checkdb\/\" \/>\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: (27\/30) use BACKUP WITH CHECKSUM to replace DBCC CHECKDB - 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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