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

[Tom Dawes-Gamble]

On Wed, 2005-01-19 at 13:38 +0000, Dickon Hood wrote:
> 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.
> 

Well I want to know how the zoning will effect the performance of my I/O
well input really since output is normally done with deferred writes.
It really doesn't matter how fast the data comes of the disk.  It's how
fast the read complete in my program that matters.

Lets say there are 15 zones on the disk and that zone 0 is twice the
speed of zone 15.  Lets assume hdparm confirms that.  So now I partition
the disk and have a file system that resides entirely in Zone 0 and
another that resides in Zone 15.  If due to OS buffering and all that
good stuff I don't see that difference in speed then what is the point?

> 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.
> 

But I'm not going to use that API in my programmes so it's not measuring
what I want to measure.  

> 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.
> 

Yes it's a shame we don't have the raw devices so we can bypass any
buffering.

The 1 gig test are probably pretty valid since I don't have a gig of
buffer cache so I'm not going to have any cache hits.

So how do I tell hdparm to test the inner or outer part of the device?

Tom.