{"id":5210,"date":"2021-04-21T11:13:45","date_gmt":"2021-04-21T18:13:45","guid":{"rendered":"https:\/\/www.sqlskills.com\/blogs\/paul\/?p=5210"},"modified":"2021-04-21T11:13:45","modified_gmt":"2021-04-21T18:13:45","slug":"small-bug-with-sample_ms-in-sys-dm_io_virtual_file_stats","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/small-bug-with-sample_ms-in-sys-dm_io_virtual_file_stats\/","title":{"rendered":"Small bug with sample_ms in sys.dm_io_virtual_file_stats"},"content":{"rendered":"<p style=\"text-align: justify;\">A prior student emailed me yesterday about some strange behavior of the\u00a0<em>sample_ms<\/em> column in\u00a0<em>sys.dm_io_virtual_file_stats<\/em>. It&#8217;s supposed to be the number of milliseconds since the SQL Server instance has been started. He has a SQL Server 2016 instance that&#8217;s been running since August 2019, and it shows the following:<\/p>\n<ul style=\"text-align: justify;\">\n<li><em>ms_ticks<\/em> from\u00a0<em>sys.dm_os_sys_info<\/em>: 51915112684 (which works out to be August 28, 2019)<\/li>\n<li><em>sample_ms<\/em> from\u00a0<em>sys.dm_io_virtual_file_stats<\/em>: 375504432 (which works out to be about 4.5 days)<\/li>\n<\/ul>\n<p style=\"text-align: justify;\">Clearly the variable inside SQL Server has wrapped\/reset a bunch of times, either at 2^31 or 2^32, even though <em>sample_ms<\/em> is supposed to be a bigint from SQL Server 2016 onward.<\/p>\n<p style=\"text-align: justify;\">We checked on a client system running 2017 and found:<\/p>\n<ul style=\"text-align: justify;\">\n<li><em>ms_ticks<\/em> from\u00a0<em>sys.dm_os_sys_info<\/em>: 7540084931<\/li>\n<li><em>sample_ms<\/em> from\u00a0<em>sys.dm_io_virtual_file_stats<\/em>: 3245117407<\/li>\n<\/ul>\n<p style=\"text-align: justify;\">In this case,\u00a0<em>sample_ms<\/em> is higher than 2^31, but has clearly wrapped\/reset, so the wrap\/reset point must be 2^32 (and 3245117407 plus 2^32 = 7540084703, almost exactly the\u00a0<em>ms_ticks<\/em> value).<\/p>\n<p style=\"text-align: justify;\">I reported this to the SQL Server team and they confirmed that it is a bug in SQL Server 2016 onward that they will fix, and also update the documentation to make it clear that in versions prior to SQL Server 2016, where <em>sample_ms<\/em> is an int, that the value will reset to zero periodically.<\/p>\n<p>Not a major bug, because if you&#8217;re doing calculations using <em>sample_ms<\/em>, you can simply substitute <em>ms_ticks<\/em> from\u00a0<em>sys.dm_os_sys_info<\/em> instead, but something some of you may want to be aware of.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>A prior student emailed me yesterday about some strange behavior of the\u00a0sample_ms column in\u00a0sys.dm_io_virtual_file_stats. It&#8217;s supposed to be the number of milliseconds since the SQL Server instance has been started. He has a SQL Server 2016 instance that&#8217;s been running since August 2019, and it shows the following: ms_ticks from\u00a0sys.dm_os_sys_info: 51915112684 (which works out to [&hellip;]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[48],"tags":[],"class_list":["post-5210","post","type-post","status-publish","format-standard","hentry","category-inside-the-storage-engine"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.5 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Small bug with sample_ms in sys.dm_io_virtual_file_stats - 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\/small-bug-with-sample_ms-in-sys-dm_io_virtual_file_stats\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Small bug with sample_ms in sys.dm_io_virtual_file_stats - Paul S. Randal\" \/>\n<meta property=\"og:description\" content=\"A prior student emailed me yesterday about some strange behavior of the\u00a0sample_ms column in\u00a0sys.dm_io_virtual_file_stats. It&#8217;s supposed to be the number of milliseconds since the SQL Server instance has been started. He has a SQL Server 2016 instance that&#8217;s been running since August 2019, and it shows the following: ms_ticks from\u00a0sys.dm_os_sys_info: 51915112684 (which works out to [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.sqlskills.com\/blogs\/paul\/small-bug-with-sample_ms-in-sys-dm_io_virtual_file_stats\/\" \/>\n<meta property=\"og:site_name\" content=\"Paul S. 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