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

Martin Hierling wrote:

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.


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