Re: Home made backup system

[rhkramer@gmail.com]

Thanks for addressing this -- I have a few questions I want to ask for my own 
edification / clarification:

On Thursday, December 26, 2019 08:18:12 AM Greg Wooledge wrote:
> The drawback of using tar is that it creates an *archive* of files -- that
> is, a single file (or byte stream) that contains a mashup of metadata and
> file contents.  If you want to extract one file from this archive, you
> have to read the entire archive from the beginning until you find the
> file you're looking for.  Remember, tar was designed for magnetic tapes,
> which are read sequentially.  It provides no way for a reader to learn
> that file xyz is at byte offset 31337 and that it should skip ahead to
> that point if it only wants that one file.

Just to confirm, I assume that is true ("no way to skip ahead to byte 31337") 
even if the underlying media is a (somewhat random access) disk instead of 
(serial access) tape?
 
> For most people, a backup using rsync to a removable *random access*
> medium (an external hard drive, or USB mass-storage device that acts
> like a hard drive) is a much better fit for their needs.

Again, I assume (I know what assume does) that "USB mass-storage device that 
acts like a hard drive" is (or might be) a pen drive type of device.  I've had 
a lot of bad luck (well, more bad luck than I'd like) with that kind of 
device, and I suspect that the problem is more likely to occur when parts of 
the device are erased to allow something new to be written to it.

In other words, I suspect it would be more reliable if it functioned a little 
bit more like a WORM (Write Once, Read Many) type device -- not that the whole 
device necessarily has to be written in one go, but more that, for highest 
reliablity,  data is appended by  writing in previously unused locations 
rather than deleting some data, and then writing new data in previously used 
and erased locations.

I once looked into the rsync type of thing (for example, I read the author's 
thesis back in the day) but I don't remember all I'd like to remember.  
(Including, I don't remember if he used the term rsync in the thesis, mabye it 
was rcopy or something.)

I don't know whether rsync, in the normal course of events will delete (erase) 
and write data in previously used locations, but it would be helpful to have 
comments, with respect to:

   * whether rsync will rewrite to previously used locations, (I think it 
does, I mean, I think under certain circumstances (maybe based on certain 
options), e.g., if a file is deleted from the "working space", that file is (or 
can be) deleted from the rsynced backup, and then that space can be reused.)

   * if when you say a "USB mass-storage device that acts like a hard drive" 
you refer to (or include) a pendrive type device

   * your experience as to the reliability of a pendrive type device, either 
in a WORM type usage (as described above) or when rewriting over previously 
used areas

Thanks!