Disk reading performance with hdparm

hdparm can be used to get disk reading performance with following command line:

• hdparm -T <dev>. Get cached disk reading performance. In fact, the page cache performance is tested.
• hdparm -t <dev>. Get raw disk reading performance.

Implementation

hdparm -T

ioctl(devfd, BLKFLSBUF, NULL);
read(devfd, buf, READING_SIZE)
do {
 seek(devfd, 0, SEEK_SET);
 read(devfd, buf, READING_SIZE);
} while (/* time is not over */);

Because the data to be read is in page cache always (if READING_SIZE << memory size), the result is page cache performance.

hdparm -t

ioctl(devfd, BLKFLSBUF, NULL);
do {
 read(devfd, buf, READING_SIZE);
} while (/* time is not over */);

Because the data to be read is not in page cache, and the bottleneck lies in raw disk reading speed, the result is raw disk reading performance.

reading prepare *         *         *
memory copy          -         -
raw disk read  ++++++++  ++++++++

As in above figure, reading reparation and memory copying in testing is neglectable, especially when readahead is considered.