Re: best way to sych a mirror SATA drive to RIAD array

John Stumbles wrote:
On Sun, 31 May 2009 20:09:51 +0000, Rahul wrote:

Further your 3 disk strategy does cover fire etc. but with periodic
tapes I also give myself the ability ti guard against inadverent
sys-admin goof-ups that might not always immediately discovered. Say, a
messed up config file that was accidently deleted and 6 months later you
want to see what the old one looked like etc?

That's why you need not just backups but point-in-time snapshots or
archives of your filesystem. The cp -rl + rsync system gives you that with
very little overhead of disk space, thanks to the magic of hard links[1],
although the space-saving will gradually degrade over time if you rename
and/or move files or directories, unless you implement some (non-trivial)
re-linking system. That's why moving older snapshots off to other media is
a good idea.

[1] arguably the lest well known and potentialy most powerful features of
unix systems, if only they were better supported (or supported at all) in
the popular user tools such as kde gui stuff. Ironically I think Windows'
NTFS supports them too, but I think you'd have to use cygwin to manipulate
them. Opens up possibilities for some amusing mischief on windoze lusers'
machines! ;-)



Hard links are *not* the most powerful features of unix systems - though they are powerful. Soft links (which NTFS and windows do not support at all) are in many ways much more useful - and are therefore much more used.

Hard links are extremely useful for backups in they way you describe - the combination "rsync && cp -al" is the basis of several backup utilities such as rsnapshot and dirvish. But they are potentially very dangerous in the hands of amateurs, because hard linked files are *not* copies. They are two names for the same file. So when a newbie makes a "fast copy" of their file using a hard link, then modifies the file, they have also modified the "backup". That's why snapshot backups are always done with "rsync" first to make a real duplicate copy, and the hardlinked files are not "live" files, but are only used for reading, deleting and renaming - never writing.
.

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