VM(ware)虚拟机中的IO与与真实的到底差多少？

突然想测试一下如果在vmware中IO的情况，给真实的IO之间的差距
1 write throughput
[root@localhost home]# time dd if=/dev/zero of=/home/vm_w bs=4096 count=1000000
1000000+0 records in
1000000+0 records out
4096000000 bytes (4.1 GB) copied, 151.627 seconds, 27.0 MB/s

real 2m33.001s
user 0m0.639s
sys 0m17.486s

2 read /write throughput

[root@localhost home]# time dd if=/home/vm_w of=/home/vm_rw bs=4096 count=1000000
1000000+0 records in
1000000+0 records out
4096000000 bytes (4.1 GB) copied, 246.963 seconds, 16.6 MB/s

real 4m7.686s
user 0m0.309s
sys 0m15.168s

3 read througput

[root@localhost home]# time dd if=/home/vm_w of=/dev/null bs=4096 count=1000000 1000000+0 records in
1000000+0 records out
4096000000 bytes (4.1 GB) copied, 93.7375 seconds, 43.7 MB/s

real 1m33.884s
user 0m0.173s
sys 0m7.655s

对比上次的数据，

[root@rac2 home]# time dd if=/dev/zero of=/home/test_write bs=4k count=1000000 [[A1000000+0 records in
1000000+0 records out
4096000000 bytes (4.1 GB) copied, 67.9907 seconds, 60.2 MB/s

real 1m11.611s
user 0m0.385s
sys 0m8.106s

第二步：read/write throughput 測試：

[root@rac2 home]# time dd if=/home/test_write of=/home/test_rw bs=4k count=1000000
1000000+0 records in
1000000+0 records out
4096000000 bytes (4.1 GB) copied, 99.2975 seconds, 41.2 MB/s

real 1m39.313s
user 0m0.437s
sys 0m10.648s
第三步， read throughput 測試：
[root@rac2 home]# time dd if=/home/test_write of=/dev/null bs=4k count=1000000
1000000+0 records in
1000000+0 records out
4096000000 bytes (4.1 GB) copied, 1.82309 seconds, 2.2 GB/s

real 0m1.824s
user 0m0.306s
sys 0m1.514s

write, read/write 都是几倍的差距可以理解，
read 怎么差距这么呢！为啥子呢！？