[sclug] disk transfer rates (was) Newbie, partitioning 120Gb HDD - recommendations?

[Dickon Hood]

On Wed, Jan 19, 2005 at 12:02:52 +0000, Tom Dawes-Gamble wrote:
: On Wed, 2005-01-19 at 10:09 +0000, Dickon Hood wrote:
: > On Wed, Jan 19, 2005 at 07:52:42 +0000, Tom Dawes-Gamble wrote:

: > [...]

: > : So I've knocked together a very rough benchmark. How long so it take to
: > : read a gigabyte from the disk.  I did each test several times and these
: > : are typical results.

: > Try hdparm -T and -t; they're more accurate than any number of runs of dd,
: > as it bypasses the OS caches.

: More accurate for what?

For producing benchmarks of the type you're trying to do.

: Personally I like using dd to measure I/O.  It's a nice userland
: programme.  Almost all the stuff I do is run in userland.  If the OS
: caches data and reads ahead such that I don't notice any difference or
: the difference is reduced.  Then I say "Well done!  Thank you".

Well, yes, of course.  And hdparm is a userland tool as well, it just uses
the exposed ATAPI API to talk to the discs directly.

If the OS is caching data, then all the timings you've just done are
useless, because they're not measuring the actual transfer speeds.
Clearly, if you're trying to measure the performance across different
parts of the platter, you want to test the raw speed, rather than which
bits the OS has or hasn't managed to cache since the last time you did
this.

-- 
Dickon Hood

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