[sclug] dd question

[James Fidell]

Quoting Rick Payne (rickp at rossfell.co.uk):
> 
> 
> --On Friday, February 21, 2003 1:26 pm +0000 Pieter Claassen 
> <pieter at openauth.co.uk> wrote:
> 
> > If I pop the master and slave HD's into a new machine booted from another
> > HD, then they copy in about 30 minutes.
> >
> > I noticed in both cases that kswapd was going for it, but other than that
> > I don't understand the massive time difference.
> 
> Presumably you had no swap on the first 'dd', so had rather less memory 
> available for caching the read/writes.

And if you are swapping, exactly which device are you swapping to, and
how will that affect disk-to-disk copy performance?

There are loads of other possibilities, too.  Perhaps the first system
has very poor IDE performance compared to the second, for any number of
reasons.  Could be the amount of read-ahead, PIO vs. DMA, other load on
the IDE bus or even motherboard hardware.  Performance problems can be
very tricky to track down, especially when you can't compare a "good"
system against a "bad" system by changing one thing at a time.

James