Tino Schwarze <backuppc.lists AT tisc DOT de> wrote on 12/08/2008 12:49:27 PM:

> > In all of the myriad of "how do I replicate a server", someone one 
time 
> > suggested using a netcat/dd like command with block-level hashing to 
copy 
> > only changed blocks.  Since then, I have searched *extensively* for 
this, 
> > and I cannot find it.
> 
> I'd just rsync the raw devices. It's actually smilar to rsync'ing DVD
> images, just a bit larger. Never checked whether that's faster -
> probably depends on your network bandwidth and I/O speeds.

You can *do* that?!?  That is taking the everything-is-a-file metaphor to 
entirely new levels...

Of course, your point is valid:  if your between-systems bandwidth 
approaches your system-to-disk bandwidth, a straight-up dd would be no 
slower, and a whole lot less work.  Only if your between-systems bandwidth 
is significantly smaller does it make sense.

You are guaranteed to take at *least* as long as a dd no matter what:  the 
rsync transfer will have to read 100% of the source disk no matter what. 
AFAICT, that is unavoidable if you're working at the block layer.  You're 
just saving any time caused by the transfer between systems.

It's too bad that there isn't a concept of "incremental" with such a 
transfer.

Another intriguing idea is to use DRDB across the Internet.  That *does* 
give you the idea of "incremental", but it's a significant and constant 
impact on bandwidth, not just at times when you want to do the transfer. 
The idea of pairing rsync and LVM snapshots is interesting.

Tim Massey


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