From the manpage:
Perform timings of cache reads for benchmark and comparison purposes. For
meaningful results, this operation should be repeated 2-3 times on an other‐
wise inactive system (no other active processes) with at least a couple of
megabytes of free memory. This displays the speed of reading directly from
the Linux buffer cache without disk access. This measurement is essentially
an indication of the throughput of the processor, cache, and memory of the
system under test.
Although you have the lowercase
-t flag first, the command is running
The important part of that message is this:
This displays the speed of reading directly from the Linux buffer cache without disk access.
This test doesn't even touch the disk. It's really just testing how fast your computer itself can perform data operations.
sudo hdparm -T /dev/sda on my machine returns:
zachary@MCServer:~$ sudo hdparm -T /dev/sda
[sudo] password for zachary:
Timing cached reads: 9270 MB in 2.00 seconds = 4637.91 MB/sec