iostat

The iostat command provides data about the I/O device usage of the system. To compute this information, seeks, data transfer completions, and the number of words transferred are counted for each disk. Iostat varies significantly between vendors. The command is normally issued with both an interval and a count specified. The interval specifies the time interval, in seconds, between records included in the output. The initial record contains cumulative statistics since system boot. It is of little value for performance tuning. Succeeding records contain data for the preceding interval. The count specifies the number of records to include in the report. The following output is from an iostat -x for Solaris

extended  device  statistics
device    r/s     w/s kr/s kw/s wait actv svc_t %w %b
  fd0     0.0     0.0 0.0  0.0   0.0  0.0  0.0   0  0
  sd3     0.0     0.0 0.0  0.0   0.0  0.0  0.0   0  0
  sd6     0.0     0.0 0.0  0.0   0.0  0.0  0.0   0  0
  st4     0.0     0.0 0.0  0.0   0.0  0.0  0.0   0  0
  nfs1    0.0     0.0 0.0  0.0   0.0  0.0  0.0   0  0
  nfs2    0.0     0.0 0.0  0.0   0.0  0.0  0.0   0  0

The main columns of interest are

wait	average number of transactions waiting for service (queue length)
actv	average number of transactions actively being serviced (removed from the queue but not yet completed)
svc_t	average service time, in milliseconds. This is the time that the disk reports that a request has been completed. If there is RAID in the way then, for example, the write confirmation will be coming from the NVRAM and not the real disk.
%w	percent of time there are transactions waiting for service (queue non-empty)
%b	percent of time the disk is busy (transactions in progress)