[RDD] Redundant Hard Drive/Backup

Cowboy curt at cwf1.com
Wed Jan 8 15:17:42 EST 2014

On Wednesday 08 January 2014 01:33:33 pm Alan Smith wrote:
> It may be a tad early yet, but I am about to the point in my comfort 
> with Rivendell to start thinking about backups.
> I know this may not be the most common way, but we have our reasons, and 
> I'd like to duplicate this behavior if I can:

 I'm certain you can.
 It's a matter of how.

> Our current automation system [MS-DOS based] contains two non-raid, but 
> mirrored hard drives.  On a schedule that we specify, it copies the new 
> files, deletes the obsolete ones, etc.

 OK, Mirror, means that the disk is a second verbatim copy of another disk.
 That's RAID-1.

 On schedule, it copies what to where ?

 If I'm understanding correctly, your "mirror" isn't really a mirror,
 but a backup copy of the working disk, periodically updated ?

 If so, then cron and rsync are your best options.
 rsync can intelligently manage updating, active files, etc.
 It can easily work as a fairly sophisticated intelligent cp, machine
 to another machine, or on the same machine.
 cron can run it every minute, to once and only once, or anything
 in between.
 Probably, I'd run rsync every 5 minutes or so.

> With my very limited knowledge of linux, I thought perhaps running dd 
> with chron might be the ticket, but coming from a Windows guy, I don't 
> know how linux behaves when attempting to copy files in use.

 dd is not a good idea.
 dd can destroy your data in one bit.
 rsync won't/can't do that.

 Any *nix copies the file as it is when the copy program gets the
 file handle.
 If something else subsequently gets a later file handle, then the first
 one isn't released or updated until the first one is released.
 Not before that moment is the file contents lost or updated, as far 
 as that process is concerned.
 Any other later process would see the newer updated file. 

 In other words, Windows creates a train wreck.
 Linux ( more correctly, the file system driver ) handles it intelligently,
 sequentially, and in proper order as one would expect.



You're never too old to become younger.
		-- Mae West

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