注：以下只是本人在VM下搭建高可用性负载的测试笔记。

1、先在VM下装一个CentOS作为NFS服务器（nfs-server.example.com），再克隆一个作为NFS客户端（nfs-client.example.com），MINI安装即可。

NFS服务器分配IP为192.168.1.100，NFS客户端分配IP为192.168.1.200。

nfs-server.example.com配置如下：

[root@nfs-server ~]# cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=nfs-server.example.com

[root@nfs-server ~]# cat /etc/hosts
# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1   localhost.localdomain localhost
192.168.1.200   nfs-client.example.com   nfs-client

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Like all database systems, PostgreSQL lets you store data using a variety of different data types. These data types allow the database engine to optimise its use of memory and storage, and to perform operations on the stored data more efficiently and with fewer errors.Data type selection plays an important role in how efficiently your RDBMS functions, and so it’s important to be fully aware of the options available to you, and to select the most appropriate data type for your storage needs. That’s where this document comes in. It outlines the most important data types supported by PostgreSQL, describing when and how each should be used, and provides you with a ready supply of choices the next time you sit down to optimise your existing databases or create new ones.
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This article was copied from http://pr.efactory.de/e-pr0.shtml, and the author is Markus Sobek.

By the end of 2001, the Google search engine introduced a new kind of penalty for websites that use questionable search engine optimization tactics: A PageRank of 0. In search engine optimization forums it is called PR0 and this term shall also be used here. Characteristically for PR0 is that all or at least a lot of pages of a website show a PageRank of 0 in the Google Toolbar, even if they do have high quality inbound links. Those pages are not completely removed from the index but they are always at the end of search results and, thus, they are hardly to be found.

A PageRank of 0 does not always mean a penalty. Sometimes, websites which seam to be penalized simply lack inbound links with an sufficiently high PageRank. But if pages of a website which have formerly been placed well in search results, suddenly show the dreaded white PageRank bar, and if there have not been any substantial changes regarding the inbound links of that website, this means – according to the prevailing opinion – certainly a penalty by Google.
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This article was copied from http://pr.efactory.de/e-further-factors.shtml, and the author is Markus Sobek.

It has been widely discussed if additional criteria beyond the link structure of the web have been implemented in the PageRank algorithm since the scientific work on PageRank has been published by Lawrence Page and Sergey Brin. Lawrence Page himself outlines the following potential influencing factors in his patent specifications for PageRank:

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This article was copied from http://pr.efactory.de/e-pagerank-yahoo.shtml, and the author is Markus Sobek.

Many experts in search engine optimization assume that certain websites obtain a special PageRank evaluation from the Google search engine which needs a manual intervention and does not derive from the PageRank algorithm directly. Mostly, the directories Yahoo and Open Directory Project (dmoz.org) are considered to get this special treatment. In the context of search engine optimization, this assumption would have the consequence that an entry into the above mentioned directories had a big impact on a site’s PageRank.

An approach that is often mentioned when talking about influencing the PageRank of certain websites is to assign higher starting values to them before the iterative computation of PageRank begins. Such proceeding shall be reviewed by looking at a simple example web consisting of two pages, whereby each of these pages solely links to the other. We assign an initial PageRank of 10 to one page and a PageRank of 1 to the other. The damping factor d is set to 0.1, because the lower d is, the faster the PageRank values converge during the iterations. So, we get the following equations for the computation of the pages’ PageRank values:
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This article was copied from http://pr.efactory.de/e-pagerank-algorithm.shtml, and the author is Markus Sobek.

Up to this point, it has been described how the number of pages and the number of inbound and outbound links, respectively, influence PageRank. Here, it will mainly be discussed in how far PageRank can be affected for the purpose of search engine optimisation by a website’s internal linking structure.

In most cases, websites are hierachically structured to a certain extend, as it is illustrated in our example of a web site consisting of the pages A, B and C. Normally, the root page is withal optimised for the most important search phrase. In our example, the optimised page A has an external inbound link from page X which has no other outbound links and a PageRank of 10. The pages B and C each receive a link from page A and link back to it. If we set the damping factor d to 0.5 the equations for the single pages’ PageRank values are given by
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This article was copied from http://pr.efactory.de/e-number-of-pages.shtml, and the author is Markus Sobek.

Since the accumulated PageRank of all pages of the web equals the total number of web pages, it follows directly that an additional web page increases the added up PageRank for all pages of the web by one. But far more interesting than the effect on the added up PageRank of the web is the impact of additional pages on the PageRank of actual websites.

To illustrate the effects of addional web pages, we take a look at a hierachically structured web site consisting of three pages A, B and C, which are joined by an additional page D on the hierarchically lower level of the site. The site has no outbound links. A link from page X which has no other outbound links and a PageRank of 10 points to page A. At a damping factor d of 0.75, the equations for the single pages’ PageRank values before adding page D are given by
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This article was copied from http://pr.efactory.de/e-pagerank-algorithm.shtml, and the author is Markus Sobek.

Since PageRank is based on the linking structure of the whole web, it is inescapable that if the inbound links of a page influence its PageRank, its outbound links do also have some impact. To illustrate the effects of outbound links, we take a look at a simple example.

We regard a web consisting of to websites, each having two web pages. One site consists of pages A and B, the other constists of pages C and D. Initially, both pages of each site solely link to each other. It is obvious that each page then has a PageRank of one. Now we add a link which points from page A to page C. At a damping factor of 0.75, we therefore get the following equations for the single pages’ PageRank values:
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This article was copied from http://pr.efactory.de/e-inbound-links.shtml, and the author is Markus Sobek.

It has already been shown that each additional inbound link for a web page always increases that page’s PageRank. Taking a look at the PageRank algorithm, which is given by

PR(A) = (1-d) + d (PR(T1)/C(T1) + … + PR(Tn)/C(Tn))

one may assume that an additional inbound link from page X increases the PageRank of page A by

d × PR(X) / C(X)

where PR(X) is the PageRank of page X and C(X) is the total number of its outbound links. But page A usually links to other pages itself. Thus, these pages get a PageRank benefit also. If these pages link back to page A, page A will have an even higher PageRank benefit from its additional inbound link.
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This article was copied from http://pr.efactory.de/e-pagerank-algorithm.shtml, and the author is Markus Sobek.

Regarding the implementation of PageRank, first of all, it is important how PageRank is integrated into the general ranking of web pages by the Google search engine. The proceedings have been described by Lawrencec Page and Sergey Brin in several publications. Initially, the ranking of web pages by the Google search engine was determined by three factors:

Page specific factors are, besides the body text, for instance the content of the title tag or the URL of the document. It is more than likely that since the publications of Page and Brin more factors have joined the ranking methods of the Google search engine. But this shall not be of interest here.
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