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From hdparm man page:

-t  Perform timings of device reads  for  benchmark  and  comparison
    purposes.   For  meaningful  results,  this  operation should be
    repeated 2-3 times on an otherwise  inactive  system  (no  other
    active  processes)  with  at least a couple of megabytes of free
    memory.  This displays the speed of reading through  the  buffer
    cache  to  the  disk  without  any  prior caching of data.  This
    measurement is an indication of how fast the drive  can  sustain
    sequential  data  reads  under  Linux,  without  any  filesystem
    overhead.  To ensure accurate measurements, the buffer cache  is
    flushed during the processing of -t using the BLKFLSBUF ioctl.

What is the "buffer cache"?

  • The hardware cache/buffer in hard drives and in some ssd drives?
  • The caching that the OS does using RAM, ie not using the cache on the harddrive. I think this is called page caching.
  • Both

Maybe it's obvious for most, but it is not for me :)

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