dd as an imaging solution.

John-Mark Gurney gurney_j at resnet.uoregon.edu
Fri Feb 9 23:18:33 UTC 2007

Sean Bryant wrote this message on Fri, Feb 09, 2007 at 14:07 -0500:
> John-Mark Gurney wrote:
> >Antony Mawer wrote this message on Tue, Feb 06, 2007 at 17:04 +1100:
> >>On 6/02/2007 1:47 PM, Sean Bryant wrote:
> >>>Dominic Marks wrote:
> >>>>Check out G4U (NetBSD based)
> >>>The only problem I can see here is that multiple parallel reads will 
> >>>have serious performance impacts, thus greatly increasing the cloning of 
> >>>the disk.
> >>>
> >>>The solution with dd, tee and netcat would just daisy chain the copy 
> >>>across the network which would be way faster.
> >>Now all you need is G4U to operate in a multicast manner like Symantec 
> >>Ghost Corporate Edition, and your transfer speed wouldn't reduce with 
> >>each additional client (eg. 100mbps for 1 client, 50mbps each for 2 
> >>clients, 33.3mbps each for 3 clients, ...)
> >
> >Add FEC to the multicast, and you can constantly stream the data, and
> >not have to worry about dropped segments as much...
> >
> Can you explain this?

FEC stands for Forward Error Correction...  Check out:

for some work that Luigi has done wrt FEC.  I've even embedded his FEC
library in the kernel w/o too much difficulty...  Wikipedia also has
an article on it...

So the idea is that you multicast out the data broken up into x packets..
In addition to x packets, you also transmit y "parity" packets...  As
long as the end system receives any combination of x and y packets where
the total unique packet count is x, you are able to reconstruct the

You choose y based upon your expected packet drop rate..  This has the
advantage that when you are transmitting the data, and a machine fails
to receive a packet, you don't have to a) retrainsmit the packet, or
b) wait till the same data packet comes along...  Because you can
replace the missed data packet w/ one of the parity packets to reconstruct
the data...

  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."

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