Poor disk performance of FreeBSD under AWS

[huanghwh]

I have a EC2 server in AWS, 4CPU+16G ram, FreeBSD 10.2R.

Use two command dd and vi:
dd if=/dev/zero of=/.swap bs=1M count=8192 & 

8589934592 bytes transferred in 117.074462 secs (73371549 bytes/sec)

when dd run in background, input vi command  to edit a small txt file "d.txt" at same time,

and then write and quit immediately:


/usr/bin/time vi d.txt


49.82 real 0.00 user 0.00 sys 

in top command show:
810 root 1 23 0 12344K 2524K wswbuf 0 0:04 5.76% dd 
821 root 1 20 0 23448K 4092K wdrain 0 0:00 0.00% vi 

vi need almost 50 seconds to quit.

any ideas?

Huang Wen Hui