[ALUG] Which way of using rsync will be faster?

[James Taylor]

On 7 Jan 2010, at 11:52, Chris G wrote:

> On Thu, Jan 07, 2010 at 11:33:04AM +0000, James Taylor wrote:
>> 
>> On 7 Jan 2010, at 11:25, Chris G wrote:
>> 
>>> On Tue, Jan 05, 2010 at 08:39:58PM +0000, steve-ALUG at hst.me.uk wrote:
>>>> Chris G wrote:
>>>>> I'm after backup rather than synchronisation so Unison doesn't really
>>>>> offer what I want.  What I'm actually doing is to rsync files to the
>>>>> backup system
>>>> Sorry - don't mean to be thick here, but surely your first step is
>>>> to ensure that your main machine and the backup machine are
>>>> synchronised, which is (AFAIK) what both Unison and rsync do.  If
>>>> you're not synchronising the two machines, then surely you'd be
>>>> using copy (cp) not rsync?
>>>> 
>>> I'm not synchronising, no changes on the backup machine will ever get
>>> back to the client machine.
>> 
>> Perhaps this is just a language/communication issue.
>> 

> Synchronising is, to my mind, bi-directional.  


I'll say again, I think this is a communication/language issue. Synchronisation is the act of making synchronous - and synchronous only means "the same", that is *after* the process both sides are the same, no matter what method it used to get there:

Whether this means:

 a) changes on one are passed to the other *and* vice-a-versa, OR
 b) if the changes are only passed from one to the other*,

is dependant upon context, and not a fixed rule. Quite often, synchronise only means one way** - consider synchronising to time servers, dns slaves, and other contexts like "manage.py syncdb" in django, both of which are one way copies, so I would be very wary of thinking that when people, programs and documentation say synchronise that this always means bi-directional.

Best Regards

James

* For completeness, just passing the changes isn't enough for a one way synch - any changes on the target would get lost in a true synch.
** One way synchronisation is trivial compared to two way - look at the difficulties many SCM apps have with merges for an example.