2

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?

  • But the connection between the motherboard and the pc is the bottleneck at 6Gb/s, and should not allow such high data rates. It can't be explained by how fast the cache on the drive is. – Mads Skjern Oct 12 '16 at 13:21
  • 1
    The cache is reads are not the disk cache. It is the kernel buffer cache, which is from RAM. The buffered disk reads are at an appropriate speed for SATA 2, which is 3Gb/s. Max MB/s speed for a SATA 3 (6Gb/s) read would be about 768 MB/s. There are several hardware and configuration factors which could affect the speed. – dobey Oct 12 '16 at 13:56
  • @Rinzwind Yes, because it's MB (bytes) per second, while 6Gbps is bits. – dobey Oct 12 '16 at 13:57
  • Could someone explain me why my question has been downvoted? – Mads Skjern Oct 12 '16 at 14:18
  • 1
    I don't think this question should be closed as opinion based or off-topic. (2 VTCs for each). It is definitely on-topic, and I really don't think it's opinion-based at all. – You'reAGitForNotUsingGit Oct 12 '16 at 15:49
4

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/

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