enabling DMA on hard drives

[Lennart Sorensen]

On Tue, Jan 18, 2005 at 09:33:35PM -0500, Anton Markov wrote:
> Actually the 'hdparm -T' command tests the speed at which data can be 
> read from the disk controller's / hard drive's cache. It represents the 
> maximum speed at which data can be transfered from the controller or the 
> hard drive itself to the CPU (I don't remember which one). Usually 
> actual disk reads are slower by at least a magnitude (factor of 10), due 
> to the limitations of the hardware. I would imagine the '-T' test is 
> useful, because certain (database) servers may benefit by using data 
> directly from the controller's cache.

Well given I get back 1700MB/s on hdparm -T and it is labeled: Timing
cached reads, I very much believe this is reading from the linux memory
cache of that device, not from the device itself.  Given the interface
to the drive is 150MB/s there is no other way to get 1700M/s even from
the disk's onboard 8MB cache.

If I turn off dual channel memory access in the BIOS, the value drops in
half.

> However, you are right in that memory management and the effectiveness 
> of the Linux I/O subsystem would probably have an impact on these test 
> results, and it could be used as a good benchmark for these systems. And 
> yes, DMA significantly improves both the physical and cache read results.

Strangely enough DMA doesn't make one bit of difference to the -T test
in my experience, which further indicates to me that it is simply a ram
speed test (as I always understood it was).

Lennart Sorensen
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