August 2019
Beginner to intermediate
232 pages
3h 50m
Chinese
Content preview from Web容量规划之美
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41
设定容量目标
看起来让人很放心，不是吗？问题是，
99.9%
个月的正常运行时间并不像你想象的
那么大：
• 30
天
= 720
小时
= 43200
分钟
• 43200
分钟 ×
99.9%= 43156.8
分钟
• 43200
分钟－
43156.8
分钟
= 43.2
分钟
这意味着，每月不能正常提供该服务的时间只要不超过
43.2
分钟，就不受惩罚条
款的制约。如果你的网站每分钟产生
3000
美元的销售收入，那你就能很容易计算
任何停机时间（连同不易衡量的顾客满意度所造成的后果）所需的费用。表
2-1
展
示了年度正常运行时间百分比。
表
2-1
：
SLA 的百分比和可接受的停机时间
在讨论关于
SLA
和可用性问题的时候经常可以听到“
5
个
9
”的条款，这就是指
99.999%
的可用性与在技术文献中一样也被广泛用于市场营销文件中。“
5
个
9
”
通常用来表明你的网站或系统被认为是高可用的。表中的正常运行时间百分比比
前面讨论过的百分比高。对于运维人员来说，使用
95%
、
99%
或者
99.99%
的可用
性指标也很常见。
这些
SLA
可用性数字的目的不仅提供对一个网站服务的信心水平，并且意味着你
可以衡量宕机时间和利润损失。我们不相信这是真正精确的，使用数学公式来证
明这一点。如果你的服务不可用达到
10
分钟，正常情况下每分钟产生
3000
美元
的利润，那你就可以假设你的业务损失是
30000
美元。在现实中，当发生（网站）
宕机时，客户也许正在选择他们不需要的东西，并且购买他们需要的东西。你的
业务也许要在客户服务方面花费一些额外的成本，来弥补宕机对收入产生的影响。 ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.