{"id":4978,"date":"2019-03-20T15:28:13","date_gmt":"2019-03-20T22:28:13","guid":{"rendered":"http:\/\/3.209.169.194\/blogs\/paul\/?p=4978"},"modified":"2019-03-20T15:28:13","modified_gmt":"2019-03-20T22:28:13","slug":"the-curious-case-of-unstoppable-dbcc-checkdb","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-unstoppable-dbcc-checkdb\/","title":{"rendered":"The Curious Case of&#8230; unstoppable DBCC CHECKDB"},"content":{"rendered":"<p style=\"text-align: justify;\"><em>(The Curious Case of\u2026<\/em>\u00a0used to be part of our bi-weekly\u00a0<a href=\"https:\/\/www.sqlskills.com\/join-the-sqlskills-insider-community\/\" target=\"_blank\" rel=\"noopener noreferrer\">newsletter<\/a>\u00a0but we decided to make it a\u00a0regular blog post instead so it can sometimes be more frequent. It covers something interesting one of us encountered when working with a client, doing some testing, or were asked in a random question from the community.)<\/p>\n<p style=\"text-align: justify;\">Last week I was asked to explain an unstoppable <em>DBCC CHECKDB<\/em> process. The client regularly runs <em>DBCC CHECKDB<\/em> at night during off-peak load times and has no problems with it but last week they ran it during the day because of a corruption indication, then decided to stop it and couldn&#8217;t. They killed the spid and nothing happened except the spid showed as being in the <em>KILLED\/ROLLBACK<\/em> state.<\/p>\n<p style=\"text-align: justify;\">Let&#8217;s back up a bit and explain what <em>DBCC CHECKDB<\/em> is doing under the covers.<\/p>\n<p style=\"text-align: justify;\">It needs to see a transactionally-consistent, unchanging view of the database. Prior to SQL Server 2005 it did this by essentially running its own version of crash recovery inside itself, by analyzing the database&#8217;s transaction log. This was pretty tortuous code that I helped write for SQL Server 2000, there were some cases where it caused false positives, and I had great fun one week in 2001 or 2002 removing all that code forever. The replacement for that code was to instead use a private database snapshot, because a database snapshot gives a transactionally-consistent, unchanging view of a database.<\/p>\n<p style=\"text-align: justify;\">So, the first thing <em>DBCC CHECKDB<\/em> does in SQL Server 2005 and later is create a database snapshot (unless you specified\u00a0<em>WITH TABLOCK<\/em>, or the target database is read-only, single-user, or already a database snapshot).<\/p>\n<p style=\"text-align: justify;\">A database snapshot runs crash recovery of the target database *into* the database snapshot, and herein lies the problem: crash recovery cannot be interrupted, and there&#8217;s no check in the crash recovery code to tell whether it&#8217;s *real* crash recovery, or crash recovery into a database snapshot (or into the <em>DBCC CHECKDB<\/em> private snapshot).<\/p>\n<p style=\"text-align: justify;\">If there&#8217;s a lot of transaction log to be recovered as part of the initial crash recovery into the database snapshot, that could take a long time. And if someone tries to kill the <em>DBCC CHECKDB<\/em> while the database snapshot is still running crash recovery, nothing will happen until the crash recovery finishes. It&#8217;s not a <em>DBCC<\/em> shortcoming, it&#8217;s a database snapshot shortcoming, and you just have to let it finish.<\/p>\n<p style=\"text-align: justify;\">So there you go &#8211; mystery explained!<\/p>\n","protected":false},"excerpt":{"rendered":"<p>(The Curious Case of\u2026\u00a0used to be part of our bi-weekly\u00a0newsletter\u00a0but we decided to make it a\u00a0regular blog post instead so it can sometimes be more frequent. It covers something interesting one of us encountered when working with a client, doing some testing, or were asked in a random question from the community.) Last week I [&hellip;]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[22,115],"tags":[],"class_list":["post-4978","post","type-post","status-publish","format-standard","hentry","category-checkdb-from-every-angle","category-the-curious-case-of"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.5 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>The Curious Case of... unstoppable DBCC CHECKDB<\/title>\n<meta name=\"description\" content=\"Why can you sometimes see an unstoppable DBCC CHECKDB?\" \/>\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\/the-curious-case-of-unstoppable-dbcc-checkdb\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The Curious Case of... unstoppable DBCC CHECKDB\" \/>\n<meta property=\"og:description\" content=\"Why can you sometimes see an unstoppable DBCC CHECKDB?\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-unstoppable-dbcc-checkdb\/\" \/>\n<meta property=\"og:site_name\" content=\"Paul S. 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