Re: Hard disk slows linux system
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On Wednesday 14 May 2003 00:33, Rus Foster wrote:
> Assuming you have got IDE disks then you want to look at hdparm. This
> can switch on DMA etc. Try running hdparm /dev/hda to see what you have
As for why increasing the speed of the HD helps, well... it doesn't.
The reason enabling DMA works is because the fall-back mode, PIO, sends all
data through the CPU (using *polling*!) instead of writing directly to
That is, with PIO there are *at least* two memory writes and reads per
request, all mediated by the CPU (not to mention polling the IDE bus).
Compare DMA, where there would be the same number of writes/reads, or half if
zero-copy is implemented, and only the read (plus one read/write w/o
zero-copy) part is done by the CPU.
No polling, either, which is what takes most of the time, really.
The reason PIO is actually faster for the HD as well is that polling the IDE
bus too much kills bandwidth/latency and modern drives *need* that, whereas
polling seldom would cause a buffer overflow in the HD...
(On a sidenote, is there any need for HDs with lots of cache on a proper OS,
with DMA enabled, which caches stuff in the much more plentiful - not to
mention faster - main RAM anyway?)
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