{"id":804,"date":"2009-08-21T19:18:00","date_gmt":"2009-08-21T19:18:00","guid":{"rendered":"\/blogs\/paul\/post\/SQL-Quiz-5-SANs-and-mirroring.aspx"},"modified":"2017-04-13T11:40:34","modified_gmt":"2017-04-13T18:40:34","slug":"sql-quiz-5-sans-and-mirroring","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/sql-quiz-5-sans-and-mirroring\/","title":{"rendered":"SQL Quiz #5: SANs and mirroring"},"content":{"rendered":"<p>\n<font face=\"verdana,geneva\" size=\"2\">There&#39;s another SQL Quiz (from <\/font><a href=\"https:\/\/chrisshaw.wordpress.com\/\"><font face=\"verdana,geneva\" size=\"2\">Chris Shaw<\/font><\/a><font face=\"verdana,geneva\" size=\"2\">) doing the rounds where people blog the answer and then tag someone. This I got tagged by two people (Jason Massie and Gail Shaw) in the same day for the same quiz (albeit over a week ago). They either think I&#39;m going to say something profound or funny, or maybe profoundly funny. Can you say something funnily profound? Ah, got it:&nbsp;profound or strange, or strangely profound, or profoundly strange. Whatever. On with quiz. I&#39;ll try not to disappoint. <\/font>\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">Question 1: Do you feel that you have a reliable SAN Solution? If so what is the secret? <\/font>\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">No. Well, that was easy, eh? <\/font>\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">Ok, seriously &#8211; we don&#39;t have a SAN or any production databases per se, as we&#39;re a training and consulting company. We do have a lot of storage hardware (3 x DELL MD3000i&#39;s packed with 26TB (unformatted)), but it&#39;s not managed by a SAN. However, we do have a lot of clients that DO have SANs. So how do we know they have a reliable SAN solution? I guess there are a number of different factors off the top of my head, and I&#39;m not a SAN expert: <\/font>\n<\/p>\n<ul>\n<li>\n<div>\n\t<font face=\"verdana,geneva\" size=\"2\">Was it designed&nbsp;for the job it&#39;s doing?&nbsp; <\/font>\n\t<\/div>\n<\/li>\n<li>\n<div>\n\t<font face=\"verdana,geneva\" size=\"2\">Are there redundant components to protect against hardware failure? <\/font>\n\t<\/div>\n<\/li>\n<li>\n<div>\n\t<font face=\"verdana,geneva\" size=\"2\">Was it configured by someone who knows what they&#39;re doing, with that brand of SAN? <\/font>\n\t<\/div>\n<\/li>\n<li>\n<div>\n\t<font face=\"verdana,geneva\" size=\"2\">Was it load tested to ensure it&#39;s can handle the job it was designed for? Was SQLIOSim run to simulate overloading the SAN to flush out any issues? <\/font>\n\t<\/div>\n<\/li>\n<li>\n<div>\n\t<font face=\"verdana,geneva\" size=\"2\">At the time it was configured, was the firmware all up-to-date, with no known bugs? I saw &#39;at the time it was configured&#39; because you have to be careful about willy-nilly upgrades to firmware in the various components. Someone that doesn&#39;t know what they&#39;re doing can destabilize a SAN by upgrading a piece of firmware that subtly changes the behavior. <\/font>\n\t<\/div>\n<\/li>\n<li>\n<div>\n\t<font face=\"verdana,geneva\" size=\"2\">Are page checksums configured on the SQL databases to help detect I\/O problems? Are regular consistency checks being run? <\/font>\n\t<\/div>\n<\/li>\n<\/ul>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">I would say that a &#39;no&#39; answer to any of these is cause for concern. <\/font>\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">Question 2: Describe database mirroring in laymen&rsquo;s terms. <\/font>\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">I&#39;ll try a few different answers. You be the judge. <\/font>\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">1) I could make this very, very simple and just say &quot;It&#39;s really technical and you don&#39;t want to know&quot;. That&#39;s not really in the spirit of things though. <\/font>\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">2) Imagine 2 seven year-old girls, in separate rooms (like my youngest daughter and one of her friends). Maybe even separate countries. Girl #1 is painting a picture, using the standard easel setup. There&#39;s a webcam pointing at the painting that girl #1 is doing. Girl #2 has a monitor and can see what girl #1 is doing. They&#39;re also on the phone with each other. Whenever girl #1 paints a brush-stroke, she can&#39;t paint any more until girl #2 has made the exact same brush-stroke and said &quot;Done it.&quot; That&#39;s the synchronous part of mirroring. Girl #1 can&#39;t get ahead of girl #2. Asynchronous mirroring is where girl #1 doesn&#39;t have to wait for girl #2 to keep up. With a witness, there&#39;s a third girl, with two webcams and another phone&#8230; <\/font>\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">Hmm &#8211; ok this analogy isn&#39;t working. It seemed so promising! Let&#39;s try again&#8230;<\/font>\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">3) This came to mind after taking a shower this morning in the hotel north of Houston. Database mirroring is like having redundant hot-water heaters. If the hot water fails from one heater, the heat-operated valve flips and the hot water is drawn from the other hot-water heater. You need two hot water heaters, and a fast-operating valve. If the first hot water heater is fixed\/warmed up again, you can manually switch the water-flow valve back. If both hot-water heaters are unavailable, no hot water. See, failover clustering won&#39;t work, because then you&#39;ve only got one hot water heater, with redundant pipes coming out of it. And replication won&#39;t work because there&#39;s a lot of latency between the water leaving the hot water heater and reaching the shower head. Ok &#8211; got a bit carried-away there.<\/font>\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">Luckily we have a redundant hotel across the street, so if there&#39;s no hot water by the time we return from a day of bird-watching on the Gulf Coast, we can move to the redundant hotel. Although that&#39;s a lot more hassle, and a lot slower than if this hotel had a redundant hot water heater.<\/font>..\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">PS Some other folks (that I know of) have replied to the quiz &#8211; here are links to their answers: <\/font>\n<\/p>\n<ul>\n<li><a href=\"http:\/\/wiseman-wiseguy.blogspot.com\/2009\/08\/sql-quiz-5-sans-star-show-must-go.html\"><font face=\"verdana,geneva\" size=\"2\">Jack Corbett<\/font><\/a><font face=\"verdana,geneva\" size=\"2\"> (<\/font><a href=\"https:\/\/twitter.com\/unclebiguns\"><font face=\"verdana,geneva\" size=\"2\">@UncleBigUns<\/font><\/a><font face=\"verdana,geneva\" size=\"2\">)<\/font><\/li>\n<li><a href=\"http:\/\/www.scarydba.com\/2009\/08\/12\/sql-quiz-5-sans-mirroring\/\"><font face=\"verdana,geneva\" size=\"2\">Grant Fritchey<\/font><\/a><font face=\"verdana,geneva\" size=\"2\"> (<\/font><a href=\"https:\/\/mobile.twitter.com\/GFritchey\"><font face=\"verdana,geneva\" size=\"2\">@GFritchey<\/font><\/a><font face=\"verdana,geneva\" size=\"2\">)<\/font><\/li>\n<li><a href=\"http:\/\/thomaslarock.com\/2009\/08\/sql-quiz-from-chris-shaw\/?utm_source=feedburner&amp;utm_medium=feed&amp;utm_campaign=Feed%3A+SQLRockstar+%28SQLRockstar%29\"><font face=\"verdana,geneva\" size=\"2\">Tom LaRock<\/font><\/a><font face=\"verdana,geneva\" size=\"2\"> (<\/font><a href=\"https:\/\/twitter.com\/SQLRockstar\"><font face=\"verdana,geneva\" size=\"2\">@SQLRockstar<\/font><\/a><font face=\"verdana,geneva\" size=\"2\">)<\/font><\/li>\n<li><a href=\"http:\/\/www.sqlservercentral.com\/blogs\/brian_kelley\/2009\/08\/17\/sql-quiz-5\/\"><font face=\"verdana,geneva\" size=\"2\">Brian Kelley<\/font><\/a><font face=\"verdana,geneva\" size=\"2\"> (<\/font><a href=\"https:\/\/twitter.com\/kbriankelley\"><font face=\"verdana,geneva\" size=\"2\">@kbriankelley<\/font><\/a><font face=\"verdana,geneva\" size=\"2\">)<\/font><\/li>\n<li><font face=\"verdana,geneva\" size=\"2\">Jason Massie<\/font><font face=\"verdana,geneva\" size=\"2\"> (<\/font><a href=\"https:\/\/twitter.com\/StatisticsIO\"><font face=\"verdana,geneva\" size=\"2\">@StatisticsIO<\/font><\/a><font face=\"verdana,geneva\" size=\"2\">)<\/font><\/li>\n<li><a href=\"http:\/\/www.brentozar.com\/archive\/2009\/08\/blog-quiz-from-chris-shaw\/?utm_source=feedburner&amp;utm_medium=feed&amp;utm_campaign=Feed%3A+BrentOzar-SqlServerDba+%28Brent+Ozar+Unlimited%29\"><font face=\"verdana,geneva\" size=\"2\">Brent Ozar<\/font><\/a><font face=\"verdana,geneva\" size=\"2\"> (<\/font><a href=\"https:\/\/twitter.com\/BrentO\"><font face=\"verdana,geneva\" size=\"2\">@BrentO<\/font><\/a><font face=\"verdana,geneva\" size=\"2\">)<\/font><\/li>\n<li><a href=\"https:\/\/chrisshaw.wordpress.com\/2009\/08\/10\/sql-quiz-5\/\"><font face=\"verdana,geneva\" size=\"2\">Chris Shaw<\/font><\/a><font face=\"verdana,geneva\" size=\"2\"> (<\/font><a href=\"https:\/\/twitter.com\/SQLShaw\"><font face=\"verdana,geneva\" size=\"2\">@SQLShaw<\/font><\/a><font face=\"verdana,geneva\" size=\"2\">)<\/font><\/li>\n<li><a href=\"http:\/\/sqlinthewild.co.za\/index.php\/2009\/08\/11\/sql-quiz-5-sans-and-magic-tricks\/?utm_source=feedburner&amp;utm_medium=feed&amp;utm_campaign=Feed%3A+SqlInTheWild+%28SQL+in+the+Wild%29\"><font face=\"verdana,geneva\" size=\"2\">Gail Shaw<\/font><\/a><\/li>\n<\/ul>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">And you can follow me on Twitter at <\/font><a href=\"https:\/\/twitter.com\/PaulRandal\"><font face=\"verdana,geneva\" size=\"2\">@PaulRandal<\/font><\/a>\n<\/p>\n<p>\n<font face=\"verdana,geneva\" size=\"2\">PPS I&#39;m not tagging anyone &#8211; been too long since the quiz started I think &#8211; the usual suspects have all been tagged already.<\/font><\/p>\n","protected":false},"excerpt":{"rendered":"<p>There&#39;s another SQL Quiz (from Chris Shaw) doing the rounds where people blog the answer and then tag someone. This I got tagged by two people (Jason Massie and Gail Shaw) in the same day for the same quiz (albeit over a week ago). They either think I&#39;m going to say something profound or funny, [&hellip;]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[32,53,54],"tags":[],"class_list":["post-804","post","type-post","status-publish","format-standard","hentry","category-database-mirroring","category-io-subsystems","category-jokes"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.5 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>SQL Quiz #5: SANs and mirroring - Paul S. 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