[arch-general] rsync for mirroring

[Nezmer at allurelinux.org]

On Sat, Oct 03, 2009 at 11:34:40AM +0530, Piyush P Kurur wrote:
> Hi,
> 
> I am part of a group of people who run a mirror for some GNU/Linux
> distros.  I have a question regarding how rsync works. The question is
> prompted by the frequent overloading of rsync mirrors. 
> 
> 
> If I am not mistaken the overall idea behind rsync is the following (I
> am assuming that dest wants to update form src)
> 
> (1) dest computes a signature of the current snapshot of its file
> system sends to src.
> 
> (2) src goes over the its current snapshot and calculates a delta
> (changes) from the signature that dest sends.
> 
> (3) src then sends the delta to dest and dest updates.
> 
> 
> Although this seems fine for a single source single destination
> system, doesint it lead to overloading of src when there are multiple
> dest, like in the case of mirroring, src has to compute the delta
> which is a time consuming process.
> 
> Alternatively if the src can do the delta computation once and for all
> then it will improve the efficiency a lot. One can imagine that a
> destination mirror can keep track of a snapshot time when it last
> synced and send the src just this. src keeps track of all the deltas,
> somewhat like rdiff-backup and then sends the dest all the deltas that
> will get it uptodate.
> 
> Regards
> 
> ppk
I never used rsync before but If I understand correctly :
rsync hashsums the files in src and dest and checks for file lists . It
removes and adds files according to the file lists and compares the
hashsums of existing files . If the hashsum is different , It replaces
the files in dest with the ones in src .

Rsync is not a binary delta implementation .