Home / ubuntu / Questions / 836054
Accepted
Mads Skjern
Asked: 2016-10-13 04:54:09 +0800 CST

My pc says it reads 4633MB/s from SSD drive cache over SATA 6Gb/s, how is that possible?

  • 772

I got this number using:

jdoe@jdoe:~$ sudo hdparm -tT /dev/sda
/dev/sda:
 Timing cached reads:   9260 MB in  2.00 seconds = 4633.30 MB/sec
 Timing buffered disk reads: 978 MB in  3.00 seconds = 325.77 MB/sec

The SSD is a Samsung 850EVO with SATA 6Gb/s (750MB/s). The laptop is an Asus S200E, which I believe also is SATA 6Gb/s.

Arent they talking about the cache on the disk? Or is it a cache on the motherboard itself?

hard-drive

1 Answers

  1. Best Answer

    From the manpage:

    -T
    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 -T first.

    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.

    Running sudo hdparm -T /dev/sda on my machine returns:

    zachary@MCServer:~$ sudo hdparm -T /dev/sda
[sudo] password for zachary:

/dev/sda:
 Timing cached reads:   9270 MB in  2.00 seconds = 4637.91 MB/sec

    http://www.linuxquestions.org/questions/linux-newbie-8/difference-between-buffered-disk-reads-and-cached-reads-891453/

    • 4