Re: [Xen-users] dd trough scp with tar

[John Haxby <john.haxby@xxxxxxxxxx>]

Martin Hierling wrote:
> Hi,
>
> fastest way is netcat. No compression overhead or anything else. you
> can reach 12MByte/s on a 100MBit/s LAN.
> on the target machine run : nc -l -p 12345 | dd of=target.img
> on the source maschine run: dd if=/dev/device | nc target 12345
>
> If you have a relly fast CPU you can also pipe it through gzip -1 on both sides.
>   
It doesn't need to be _that_ fast: you should be able to compress 
on-the-fly faster than that for any machine made in the last few years, 
you'd only need something really fast for a gigabit network and then 
you're still more likely to be limited by disk bandwidth than cpu speed.
Having said that, netcat does have a very low overhead and is a good 
choice for transferring large files over the net.  If you have firewall 
problems or security is a concern though, then "scp -C" (to enable 
compression) is also a very good choice and "scp -C remotehost:/dev/sda1 
local.img" is actually pretty simple and will work quite well.
Whether the disk image compresses well depends on what is in 
corresponding file system: if it's a fairly new and fairly large disk 
then chances are that most of the blocks are full of zeroes which 
compress very well :-)  If it's a disk practically full of, say, video 
files then it won't noticeably compress.   In other words, you'll get 
somewhere between 0.1% and 99.9% compression depending what's on the 
disk -- and it's hard to predict what you'll actually get.
Finally, you may need to take into account the network reliability.  The 
larger the disk image and the slower (or more distant) the link, the 
more likely something will drop the connection part way through.  If 
this is a problem you may wish to look at the count, skip and seek dd 
options.
jch

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