Raid vs rsync -

[Gordon Messmer]

On 03/09/2015 11:04 AM, Bob Goodwin wrote:
> However I have been wondering if it wouldn't work just as well to
> periodically rsync the drive in use with a second drive?

I know I'm going to repeat some of what has already been said.  My 2c 
anyway:

No, rsync would not work just as well.

Do you want your system to continue functioning when one of your drives 
fails? If so, then set up RAID1 and make sure you actually get and read 
email from cron jobs.  In the event of failure, the mdmonitor service 
will send email to "root" to indicate that a drive needs to be replaced. 
  The down side is that your data will have no backups.  If a file is 
accidentally deleted or corrupted, you probably have no recourse.

Do you, instead, want multiple levels of online backups?  In that case, 
there are a handful of backup applications, including rsnapshot, that 
handle rotation and rsync to provide efficient backups.  If your primary 
drive fails, you'll deal with the outage while you get a replacement 
drive, format it, install a system, restore your data, etc, which could 
be a fairly long process.  Instead, you'll gain very coarse file 
versions and protection from accidental deletions.

> Am I going wrong somewhere in my thinking?

Thinking that you have to make a choice between the two may be in error. 
  Depending on the size of your disks and the amount of data on them, 
you could potentially have both RAID1 and backups.

Build a system with a RAID1 mirror on the two drives that uses half of 
the available space for your system, and half of the space for a 
separate backup filesystem.  Keeping the backup filesystem separate 
provides some additional protection against filesystem corruption.  It's 
still possible for some errors to destroy both your system and its 
backups, but in most cases, you'll get good coverage for the most common 
failures with this setup.