{"id":3655,"date":"2018-08-11T23:16:03","date_gmt":"2018-08-11T15:16:03","guid":{"rendered":"http:\/\/www.brofive.org\/?p=3655"},"modified":"2018-08-11T23:28:16","modified_gmt":"2018-08-11T15:28:16","slug":"%e5%ae%8c%e6%95%b4%e7%9a%84%e5%8f%af%e7%94%a8%e6%80%a7%e5%ae%9a%e4%b9%89%ef%bc%9a%e4%bb%8e9%e4%b8%aa5%e5%88%b09%e4%b8%aa9","status":"publish","type":"post","link":"http:\/\/www.brofive.net\/?p=3655","title":{"rendered":"\u5b8c\u6574\u7684\u53ef\u7528\u6027\u5b9a\u4e49\uff1a\u4ece9\u4e2a5\u52309\u4e2a9"},"content":{"rendered":"<p><span style=\"font-size: large;\">\u5e38\u89c1\u7684\u53ef\u7528\u6027\u6307\u6807\u57fa\u672c\u4e0a\u90fd\u5217\u5728\u8fd9\u91cc\u4e86\uff0c\u5176\u4e2d3N5\u662f\u516c\u6709\u4e91\u7684\u865a\u62df\u673a\u53ef\u7528\u6027\uff0c5N\u662f\u7535\u4fe1\u7ea7\u53ef\u7528\u6027\u3002\u4f46\u662f\u5728\u4e91\u5b58\u50a8\u4e2d\u89c1\u5230\u768411\u4e2a9,\u6307\u7684\u662f\u53ef\u9760\u6027\uff0c\u5e76\u975e\u53ef\u7528\u6027\uff0c\u6ca1\u6709\u5217\u5728\u8fd9\u91cc\u4e5f\u662f\u6b63\u5e38\uff0c\u5426\u5219\u4e5f\u592a\u9ad8\u4e86\u3002<\/span><\/p>\n<p><a href=\"http:\/\/www.brofive.net\/wp-content\/uploads\/2018\/08\/image-25.png\"><span style=\"font-size: large;\"><img loading=\"lazy\" decoding=\"async\" style=\"display: inline; border: 0px;\" title=\"image\" src=\"http:\/\/www.brofive.net\/wp-content\/uploads\/2018\/08\/image_thumb-14.png\" alt=\"image\" width=\"900\" height=\"497\" border=\"0\" \/><\/span><\/a><\/p>\n<p>&nbsp;<\/p>\n<p><span style=\"font-size: large;\">Percentages of a particular order of magnitude are sometimes referred to by the number of nines or &#8220;class of nines&#8221; in the digits. For example, electricity that is delivered without interruptions (blackouts, brownouts or surges) 99.999% of the time would have 5 nines reliability, or class five. In particular, the term is used in connection with mainframes or enterprise computing, often as part of a service-level agreement.<\/span><\/p>\n<p><span style=\"font-size: large;\">Similarly, percentages ending in a 5 have conventional names, traditionally the number of nines, then &#8220;five&#8221;, so 99.95% is &#8220;three nines five&#8221;, abbreviated 3N5.<sup><a href=\"https:\/\/en.wikipedia.org\/wiki\/High_availability#cite_note-6\">\u00a0<\/a><\/sup> This is casually referred to as &#8220;three and a half nines&#8221;, but this is incorrect: a 5 is only a factor of 2, while a 9 is a factor of 10, so a 5 is 0.3 nines: 99.95% availability is 3.3 nines, not 3.5 nines. More simply, going from 99.9% availability to 99.95% availability is a factor of 2 (0.1% to 0.05% unavailability), but going from 99.95% to 99.99% availability is a factor of 5 (0.05% to 0.01% unavailability), over twice as much.<\/span><\/p>\n<p><span style=\"font-size: large;\">A formulation of the <i>class of 9s<\/i> <strong><em>c<\/em><\/strong>\uff0cbased on a system&#8217;s unavailability\u00a0 <strong><em>x<\/em><\/strong> would be<\/span><\/p>\n<p><a href=\"http:\/\/www.brofive.net\/wp-content\/uploads\/2018\/08\/image-26.png\"><span style=\"font-size: large;\"><img loading=\"lazy\" decoding=\"async\" style=\"display: inline; border: 0px;\" title=\"image\" src=\"http:\/\/www.brofive.net\/wp-content\/uploads\/2018\/08\/image_thumb-15.png\" alt=\"image\" width=\"231\" height=\"53\" border=\"0\" \/><\/span><\/a><\/p>\n<p><span style=\"font-size: large;\">(cf. Floor and ceiling functions).<\/span><\/p>\n<p><span style=\"font-size: large;\">A similar measurement is sometimes used to describe the purity of substances.<\/span><\/p>\n<p><span style=\"font-size: large;\">In general, the number of nines is not often used by a network engineer when modeling and measuring availability because it is hard to apply in formula. More often, the unavailability expressed as a probability (like 0.00001), or a downtime per year is quoted. Availability specified as a number of nines is often seen in <\/span><span style=\"font-size: large;\">marketing<\/span><span style=\"font-size: large;\"> documents. The use of the &#8220;nines&#8221; has been called into question, since it does not appropriately reflect that the impact of unavailability varies with its time of occurrence. For large amounts of 9s, the &#8220;unavailability&#8221; index (measure of downtime rather than uptime) is easier to handle. For example, this is why an &#8220;unavailability&#8221; rather than availability metric is used in hard disk or data link bit error rates.<\/span><\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u5e38\u89c1\u7684\u53ef\u7528\u6027\u6307\u6807\u57fa\u672c\u4e0a\u90fd\u5217\u5728\u8fd9\u91cc\u4e86\uff0c\u5176\u4e2d3N5\u662f\u516c\u6709\u4e91\u7684\u865a\u62df\u673a\u53ef\u7528\u6027\uff0c5N\u662f\u7535\u4fe1\u7ea7\u53ef\u7528\u6027\u3002\u4f46\u662f\u5728\u4e91\u5b58\u50a8\u4e2d\u89c1\u5230\u7684&#8230;<\/p>\n","protected":false},"author":1,"featured_media":3652,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[22,284,128,308],"tags":[463],"views":8098,"_links":{"self":[{"href":"http:\/\/www.brofive.net\/index.php?rest_route=\/wp\/v2\/posts\/3655"}],"collection":[{"href":"http:\/\/www.brofive.net\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.brofive.net\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.brofive.net\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.brofive.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3655"}],"version-history":[{"count":2,"href":"http:\/\/www.brofive.net\/index.php?rest_route=\/wp\/v2\/posts\/3655\/revisions"}],"predecessor-version":[{"id":3657,"href":"http:\/\/www.brofive.net\/index.php?rest_route=\/wp\/v2\/posts\/3655\/revisions\/3657"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.brofive.net\/index.php?rest_route=\/wp\/v2\/media\/3652"}],"wp:attachment":[{"href":"http:\/\/www.brofive.net\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3655"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.brofive.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3655"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.brofive.net\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3655"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}