> IBM techies discussed some interesting ideas recently. One idea is just to transmit >changes. > If you have mirrored disks that are geographically separated, you can look at the >old record > on the driving system before you replace it with the new. XORing a changed 4KB >block with the > original usually produces just a few dozen bytes of changes - with a compressing >tranmission > channel the binary zeroes (where the data was unchanged) all compress away. >Transmit the > result of the XOR and XOR it with the remote copy to recreate the changed block.
The intermezzo file system has similar ideas but uses the rsync algorithm to transfer only the changed parts of the file. Since rsync can describe changes compactly and also describe movement and shuffling of pieces of data its extremely efficient
