статистика по блочным устройствам в linux

попробуй sar.
sar - Collect, report, or save system activity information.

/proc/diskstats - вроде то что тебе нужно.

sysstat еще

iostat, точнее

В sysfs есть файлики /sys/block/<dev>/stat по каждому из блочных устройств.
Там в том числе есть и счётчик записанного/считанного - в 512-байтных секторах.
Цитата из Documentation/block/stat.txt (в исходниках ядра):

The stat file consists of a single line of text containing 11 decimal
values separated by whitespace. The fields are summarized in the
following table, and described in more detail below.

Name units description
---- ----- -----------
read I/Os requests number of read I/Os processed
read merges requests number of read I/Os merged with in-queue I/O
read sectors sectors number of sectors read
read ticks milliseconds total wait time for read requests
write I/Os requests number of write I/Os processed
write merges requests number of write I/Os merged with in-queue I/O
write sectors sectors number of sectors written
write ticks milliseconds total wait time for write requests
in_flight requests number of I/Os currently in flight
io_ticks milliseconds total time this block device has been active
time_in_queue milliseconds total wait time for all requests