可能是linux内核bug的缘故，iostat -x 可能会显示大量的I/O反馈时间（svctm)，和100%的
带宽使用率（%util),对于某些设备。实际上这些设备在/proc/{partitions,diskstats}字段#9
有一个值，通常不是0，有时候甚至是负值。然而这个字段值应该是0，因为它提供正在运
行IO的数量（当请求被接收的时候自增，当完成的时候自减） .
摘自：http://linuxcommand.org/man_pages/iostat1.html
翻译原文：
/proc filesystem must be mounted for iostat to work.
Extended statistics are available only with post 2.5 kernels.
Because of what seems to be a Linux kernel bug, iostat -x may display
huge I/O response times (svctm) and a bandwidth utilization (%util) of
100% for some devices. Indeed these devices have a value for the field
#9 in /proc/{partitions,diskstats} which is always different from 0,
and even negative sometimes. Yet this field should go to zero, since it
gives the number of I/Os currently in progress (it is incremented as
requests are submitted, and decremented as they finish).