[Beowulf] small file systems

Mark Hahn hahn at mcmaster.ca
Wed Aug 29 23:29:03 EDT 2007


> isn't have 10 slower 'datawulf' nodes better than 1 at the same speed
> (with them added up) for data access? surely the optimal speed is a
> processor that can handle the intensive IO streaming and a kernel + a bit
> of overhead for good luck.

you can never just add up speed like that - single-fast will always 
have different properties from several-slow.

> another idea i have floating - with lower speed cpu's eg: 500mhz or even
> up to 1066mhz - cant you use 'normal' as you l1 cache??? surely that is

of course not, since dram is limited to ~50ns at least as currently
conceived.  that would correspond with a cpu clock of around 20 MHz.

> good for data steaming eg: a 1GHz cpu with 1024MB l1 cache, a further 3GB

as a crude estimate, L1 cache takes ~6 transistors per bit, so 
1G cache would correspond with 50 billion transistors, which is 
much larger than current chips.  such a large L2 would also not
run at 1GHz, since there is always a size-speed tradeoff.

> running at 400MHz, attached to 4 sata 2 drives?

3GB is overkill for streaming.  you want your network, memory and disk
bandwidths to match, with modest buffering.  if you had a 4x raid0,
you would hope to achieve around 200 MB/s, so 3GB would be an absurdly
large (15 second) buffer.
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