07-03-2012
Just because it's a raw disk device doesn't mean you're actually reading the disk raw. Disk cache and read-ahead gets used for it just like everything else.
Try hdparm for some more direct tests.
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RK(4) Kernel Interfaces Manual RK(4)
NAME
rk - RK-11/RK03 or RK05 disk
DESCRIPTION
Rk? refers to an entire disk as a single sequentially-addressed file. Its 256-word blocks are numbered 0 to 4871. Minor device numbers
are drive numbers on one controller.
The rk files discussed above access the disk via the system's normal buffering mechanism and may be read and written without regard to
physical disk records. There is also a `raw' interface which provides for direct transmission between the disk and the user's read or
write buffer. A single read or write call results in exactly one I/O operation and therefore raw I/O is considerably more efficient when
many words are transmitted. The names of the raw RK files begin with rrk and end with a number which selects the same disk as the corre-
sponding rk file.
In raw I/O the buffer must begin on a word boundary, and counts should be a multiple of 512 bytes (a disk block). Likewise seek calls
should specify a multiple of 512 bytes.
FILES
/dev/rk?, /dev/rrk?
BUGS
In raw I/O read and write(2) truncate file offsets to 512-byte block boundaries, and write scribbles on the tail of incomplete blocks.
Thus, in programs that are likely to access raw devices, read, write and lseek(2) should always deal in 512-byte multiples.
RK(4)