[Info-vax] BACKUP, rsync, Time Machine (was: Re: Re; Spiralog, RMS Journaling...)

[lawrencedo99 at gmail.com]

On Tuesday, June 21, 2016 at 3:38:50 AM UTC+12, Paul Sture wrote:
> Paul:
>>> Time Machine uses a File System Event store to track changed directories.
> 
> Lawrence: 
> 
>> rsync doesn’t need one.
> 
> You underestimate the size of the problem.  When not much has changed, an
> hourly Time Machine run will complete in a minute or two.
> 
> rsync cannot possibly scan all the directories on a system for changes in
> such a short time, at least not on the spinning rust I have here.

It normally takes a shortcut: if the source and destination files have exactly the same size and the same timestamps, then it assumes they are the same. So yes, rsync can do fast scans.

There is a --checksum option if you want to disable this. Which I use as an extra check, for example if I am going to wipe the original disk to repartition it before restoring.

> Want timings?

Not on HFS-Plus, no. We all know the quality of Apple’s filesystems <http://www.zdnet.com/article/horrors-linus-torvalds-calls-hfs-utter-crap/> ...