{"id":5161,"date":"2020-10-28T11:01:50","date_gmt":"2020-10-28T18:01:50","guid":{"rendered":"http:\/\/3.209.169.194\/blogs\/paul\/?p=5161"},"modified":"2020-10-28T11:13:30","modified_gmt":"2020-10-28T18:13:30","slug":"the-curious-case-of-the-aborted-xdes-map","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-the-aborted-xdes-map\/","title":{"rendered":"The Curious Case of&#8230; the aborted xdes map"},"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 emailed a question about weird messages in someone&#8217;s error log on an Azure Managed Instance, similar to these:<\/p>\n<pre class=\"brush: plain; gutter: false; title: ; toolbar: true; wrap-lines: true; notranslate\" title=\"\">\r\n&#x5B;DbId:12]. Removing xdes id: 0000:8112c9d0 from aborted xdes map.\r\n&#x5B;DbId:12]. Removing xdes id: 0000:8112b45e from aborted xdes map.\r\n&#x5B;DbId:12]. Removing xdes id: 0000:8112b403 from aborted xdes map.\r\n<\/pre>\n<p style=\"text-align: justify;\">First off, an XDES is a data structure (and C++ class) in the Storage Engine that represents a transaction; it stands for &#8216;transaction descriptor&#8217;. The XDES ID is the same one that you see in the <em>Transaction ID<\/em> column in the output from the <em>fn_dblog<\/em> function for examining transaction log contents (see <a href=\"https:\/\/www.sqlskills.com\/blogs\/paul\/using-fn_dblog-fn_dump_dblog-and-restoring-with-stopbeforemark-to-an-lsn\/\" target=\"_blank\" rel=\"noopener noreferrer\">here<\/a> for examples).<\/p>\n<p style=\"text-align: justify;\">The messages are benign and are to do with the internal workings of the Accelerated Database Recovery feature, that allows instantaneous rollback of transactions through a clever versioning mechanism.<\/p>\n<p style=\"text-align: justify;\">The aborted transaction map (ATM) contains a list of transactions that have rolled back (internally called &#8216;aborted&#8217;), but their effects have not yet been &#8216;removed&#8217; from the database. The ATM is used:<\/p>\n<ul style=\"text-align: justify;\">\n<li>when a query reads a row, to see whether the current version of the row is from a transaction that subsequently rolled back, and so a previous version of the row should be used<\/li>\n<li>when a query is updating a row, to see whether the current version of the row is from a transaction that subsequently rolled back, so the updated row should overwrite the current version of the row<\/li>\n<\/ul>\n<p style=\"text-align: justify;\">When the effects of a rolled-back transaction no longer need to be considered, the transaction ID is removed from the ATM.<\/p>\n<p>You can read a great post that explains the internals of Accelerated Database Recovery <a href=\"https:\/\/www.red-gate.com\/simple-talk\/sql\/database-administration\/how-does-accelerated-database-recovery-work\/\" target=\"_blank\" rel=\"noopener noreferrer\">here<\/a>.<\/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":[48,115,98],"tags":[],"class_list":["post-5161","post","type-post","status-publish","format-standard","hentry","category-inside-the-storage-engine","category-the-curious-case-of","category-transaction-log"],"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... the aborted xdes map - 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\/the-curious-case-of-the-aborted-xdes-map\/\" \/>\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... the aborted xdes map - Paul S. Randal\" \/>\n<meta property=\"og:description\" content=\"(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.) 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