nginx机器配置：
CPU： E7500 @ 2.93GHz (双核)
内存：4G
varnish机器配置：
CPU： E3300 @ 2.50GHz (双核)
内存：2G
测试脚本执行机器配置：
CPU： AMD Phenom(tm) II X4 925 Processor
内存：2G

2.1：安装siege

使用压力测试工具siege（http://www.joedog.org/index/siege-home） 来验证。可以根据路径来并发重复压力测试。安装方式：
下载：wget ftp://sid.joedog.org/pub/siege/siege-latest.tar.gz
安装：
#./configure
#make && make install

[root@localhost test] siege -c 100 -r 100 -f varnish_url.txt
[root@localhost test] siege -c 100 -r 100 -f nginx_url.txt

参数说明：
-c 100 并发100个用户
-r 100 重复循环100次
-f varnish_url.txt 任务列表：URL列表

2.2：构造URL
构造了536个url测试，其中1个是错误的url。

到测试机器上,

[root@localhost test] cd /data/htdocs/resources/web
[root@localhost test] find . -name "*.jpg" -print > varnish_url.txt

2.3：开启nginx页面缓存方法
开启nginx的缓存图片的方法，在nginx的conf中添加：

location ~ .*\.(gif|jpg|jpeg|png|bmp|swf|ico)$ {
    expires      30d;
    access_log   off;
}
感谢鱼哥 @smallfishxy 指导

3.1.1：nginx开启页面缓存的情况

Transactions: 10000 hits
Availability: 100.00 %
Elapsed time: 65.83 secs
Data transferred: 260.83 MB
Response time: 0.01 secs
Transaction rate: 151.91 trans/sec
Throughput: 3.96 MB/sec
Concurrency: 2.06
Successful transactions: 9900
Failed transactions: 0
Longest transaction: 3.03
Shortest transaction: 0.00

3.1.2：nginx不开启页面缓存的情况
Transactions: 10000 hits
Availability: 100.00 %
Elapsed time: 63.90 secs
Data transferred: 266.10 MB
Response time: 0.01 secs
Transaction rate: 156.49 trans/sec
Throughput: 4.16 MB/sec
Concurrency: 2.18
Successful transactions: 9900
Failed transactions: 0
Longest transaction: 0.35
Shortest transaction: 0.00

3.1.3：nginx开启页面缓存第二次测试的情况
Transactions: 10000 hits
Availability: 100.00 %
Elapsed time: 63.05 secs
Data transferred: 266.10 MB
Response time: 0.01 secs
Transaction rate: 158.60 trans/sec
Throughput: 4.22 MB/sec
Concurrency: 2.06
Successful transactions: 9900
Failed transactions: 0
Longest transaction: 0.34
Shortest transaction: 0.00

3.2.1：varnish清空缓存第一次测试情况
Transactions: 10000 hits
Availability: 100.00 %
Elapsed time: 62.87 secs
Data transferred: 266.10 MB
Response time: 0.01 secs
Transaction rate: 159.06 trans/sec
Throughput: 4.23 MB/sec
Concurrency: 1.95
Successful transactions: 9900
Failed transactions: 0
Longest transaction: 0.37
Shortest transaction: 0.00

3.2.2：varnish有缓存测试
Transactions: 10000 hits
Availability: 100.00 %
Elapsed time: 61.91 secs
Data transferred: 266.10 MB
Response time: 0.01 secs
Transaction rate: 161.52 trans/sec
Throughput: 4.30 MB/sec
Concurrency: 1.78
Successful transactions: 9900
Failed transactions: 0
Longest transaction: 0.34
Shortest transaction: 0.00

3.2.3：varnish负责均衡性能
Transactions: 10000 hits
Availability: 100.00 %
Elapsed time: 59.75 secs
Data transferred: 266.10 MB
Response time: 0.01 secs
Transaction rate: 167.36 trans/sec
Throughput: 4.45 MB/sec
Concurrency: 1.98
Successful transactions: 9900
Failed transactions: 0
Longest transaction: 3.01
Shortest transaction: 0.00

• 在varnish机器性能较差的情况下，在高并发情况下，能够跑出比nginx更好的性能。
• 线上是读取了单台图片服务器，使用varnish能够解决图片服务负责均衡的问题，减少了nginx对内存的存储双份造成浪费的问题。
• 有varnish负载均衡，单台img服务器故障时能让服务继续。当然varnish故障了，需要有备份服务器。