{"id":4970,"date":"2019-03-06T15:35:18","date_gmt":"2019-03-06T23:35:18","guid":{"rendered":"http:\/\/3.209.169.194\/blogs\/paul\/?p=4970"},"modified":"2019-03-12T16:59:35","modified_gmt":"2019-03-12T23:59:35","slug":"the-curious-case-of-faster-disks-causing-more-writelog-waits","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-faster-disks-causing-more-writelog-waits\/","title":{"rendered":"The Curious Case of&#8230; faster disks causing more WRITELOG waits"},"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;\">I was contacted last week by someone who was asking how are faster disks causing more\u00a0<a href=\"https:\/\/www.sqlskills.com\/help\/waits\/writelog\/\" target=\"_blank\" rel=\"noopener noreferrer\"><em>WRITELOG<\/em><\/a>\u00a0waits. They were seeing lots of these waits, with an average wait time of 18ms. The log was stored on a Raid-1 array, using locally-attached spinning disks in the server. They figured that by moving the log to Raid-1 array of SSDs, they&#8217;d reduce the\u00a0<em>WRITELOG<\/em> wait time and get better workload throughput.<\/p>\n<p style=\"text-align: justify;\">They did so and got better performance, but were very surprised to now see\u00a0<em>WRITELOG<\/em> as\u00a0the most frequent\u00a0wait type on the server, even though the average wait time was less than 1ms, and asked me to explain.<\/p>\n<p style=\"text-align: justify;\">Let me start by saying this is not unusual, and I&#8217;ve seen this sequence of events many times with clients and students.<\/p>\n<p style=\"text-align: justify;\">A\u00a0<em>WRITELOG<\/em> wait occurs when a log block is being flushed to disk, and the two most common causes of this are that the log block is full (60KB) or a transaction committed. If it&#8217;s the case that a workload is comprised of lots of very small transactions, then a reduction in wait time for the log block flush from 18ms down to less than 1ms is very likely to result in a higher throughput. This means more transactions happening and hence more log block flushes when the transactions commit, so *more*\u00a0<em>WRITELOG<\/em> waits.<\/p>\n<p style=\"text-align: justify;\"><strong>Bottom line:\u00a0<\/strong>What was confusing to them was that performance tuning is usually about reducing the incidence of waits and\/or the individual wait times. This is about the only example I can think of where you do something that increases performance and the wait type you were concerned about becomes more prevalent.<\/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.) I was contacted [&hellip;]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[115,98,101],"tags":[],"class_list":["post-4970","post","type-post","status-publish","format-standard","hentry","category-the-curious-case-of","category-transaction-log","category-wait-stats"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.5 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Why are faster disks causing more WRITELOG waits?<\/title>\n<meta name=\"description\" content=\"Explanation of faster disks causing more WRITELOG waits\" \/>\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-faster-disks-causing-more-writelog-waits\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Why are faster disks causing more WRITELOG waits?\" \/>\n<meta property=\"og:description\" content=\"Explanation of faster disks causing more WRITELOG waits\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-faster-disks-causing-more-writelog-waits\/\" \/>\n<meta property=\"og:site_name\" content=\"Paul S. Randal\" \/>\n<meta property=\"article:published_time\" content=\"2019-03-06T23:35:18+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2019-03-12T23:59:35+00:00\" \/>\n<meta name=\"author\" content=\"Paul Randal\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Paul Randal\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"2 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"WebPage\",\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-faster-disks-causing-more-writelog-waits\/\",\"url\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-faster-disks-causing-more-writelog-waits\/\",\"name\":\"Why are faster disks causing more WRITELOG waits?\",\"isPartOf\":{\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/#website\"},\"datePublished\":\"2019-03-06T23:35:18+00:00\",\"dateModified\":\"2019-03-12T23:59:35+00:00\",\"author\":{\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/#\/schema\/person\/ffcec826c18782e1e0adf173826a7fce\"},\"description\":\"Explanation of faster disks causing more WRITELOG waits\",\"breadcrumb\":{\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-faster-disks-causing-more-writelog-waits\/#breadcrumb\"},\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"ReadAction\",\"target\":[\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-faster-disks-causing-more-writelog-waits\/\"]}]},{\"@type\":\"BreadcrumbList\",\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/the-curious-case-of-faster-disks-causing-more-writelog-waits\/#breadcrumb\",\"itemListElement\":[{\"@type\":\"ListItem\",\"position\":1,\"name\":\"Home\",\"item\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/\"},{\"@type\":\"ListItem\",\"position\":2,\"name\":\"The Curious Case of&#8230; faster disks causing more WRITELOG waits\"}]},{\"@type\":\"WebSite\",\"@id\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/#website\",\"url\":\"https:\/\/www.sqlskills.com\/blogs\/paul\/\",\"name\":\"Paul S. 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