Most Linux admins are not aware of hidden switch (undocumented switch) called -D. The -D option display a nice summery of disk I/O subsystem since boot time. Output includes total time spent reading and writing data, merged reads and merged writes (kernel disk i/o optimization technique) and other parameters.

vmstat command with -D option

Type the command at shell prompt:
vmstat -D
Output:

           30 disks
            8 partitions
     13739406 total reads
      3887474 merged reads
   1180260353 read sectors
    109630647 milli reading
    118650080 writes
    250115464 merged writes
   2950543038 written sectors
   2382220771 milli writing
            0 inprogress IO
       113832 milli spent IO

This is my local backup server hosted at local data center:
$ uptime
Output:

12:15:07 up 33 days,  9:15,  2 users,  load average: 1.71, 1.49, 1.03

Since last 33 days my server spent total 1180260353 milliseconds reading disk. To get more detailed about disk I/O stats, use the -d option as described in previous article – How do I find out Linux Disk utilization?