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

[Stephen Hoffman]

On 2016-06-19 01:44:35 +0000, lawrencedo99 at gmail.com said:

> The funny thing is, rsync can perform mirror backups with deduping of 
> unchanged files, without messing around with filesystem integrity; why 
> couldn’t Apple have done the same?

rsync is somewhat analogous to Time Machine in that both copy files 
around.  But the two differ.  rsync is a remote synchronization tool.  
Time Machine is a backup tool, and maintains months or years of 
previous versions of files over one or more local or remote backup 
target volumes, as well as providing the ability to restore and boot 
the recovered volume(s), among other differences from rsync.  TM avoids 
storing duplicate copies of a file, though does not provide the file or 
sector-level de-dupling available with some storage systems.  Having 
used both, TM and rsync are different targets and different users and 
different implementations.

As for the integrity, Time Machine solves the problem that the vast 
majority of its users has.  That the vast majority of OpenVMS users 
have, but have to write their own DCL procedures for.

Features of APFS provide capabilities that would appear to benefit Time 
Machine, so it'll be interesting to see how that's integrated.  TM is 
one of the last consumers of AFP on macOS systems, and it's very likely 
it'll migrate to SMB in some upcoming release.  OpenVMS BACKUP, by 
contrast, works very well but has limited remote access — other than 
via an existing cluster and its storage access or via DECnet FAL, or 
via some unsupported DECnet hackery*, and none those lend themselves to 
easy restoration and recovery.  BACKUP does have compression and Time 
Machine does not, but BACKUP lacks de-duping, and the incremental 
support within BACKUP is rather more complex to implement than it 
should be.

*unsupported DECnet hackery: http://labs.hoffmanlabs.com/node/598



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