If they are not both available on the same machine, these algorithms cannot be used. Thus they require that both files are available beforehand at one end of the link. The problem is that the normal methods for creating a set of differences between two files rely on being able to read both files. A common method is to send just the differences between file_A and file_B down the link and then use such list of differences to reconstruct the file on the remote end. Now assume that file_A and file_B are quite similar, and to speed things up, you take advantage of this similarity. To make it more efficient, you could compress file_A before sending it, but that would usually only gain a factor of 2 to 4. If file_A is large, copying it onto file_B will be slow, and sometimes not even possible. Now imagine that the two files are on two different servers connected by a slow communications link, for example, a dial-up IP link. The obvious method is to copy file_A onto file_B. You wish to update file_B to be the same as file_A. Imagine you have two files, file_A and file_B. Rsync is a tool was created by Andrew Tridgell and Paul Mackerras who were motivated by the following problem: This article provides some further information about rsync, and an explanation of what happened in that story. However, what most of us forget is to understand what rsync really is, and how is it used, and the most important in my opinion is, where it come from. The friend believed that rsync is a magic tool that should just “sync” the file as it is. They could not understand why the file they copied was 10GB on site A but but it became 100GB on-site B. Some years ago, a friend who used to work on my team needed to copy virtual machine templates from site A to site B. The problem is when we run things without understanding them. We all do it, and the copy-and-paste itself is not a problem. There is a notion that a lot of people working in the IT industry often copy and paste from internet howtos.
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