Re: Best way to duplicate HDs

["Jason Lim" <maillist@jasonlim.com>]

> > I know of a few hardware solutions that do something like this, but
would
> > like to do this in hardware. They claim to perform a "mirror" of one
HD to
> > another HD while the system is live and in use.
>
> It's called RAID-1.

I dunno... whenever I think of "RAID" I always think of live mirrors that
operate constantly and not a "once in a while" mirror operation just to
perform a backup (when talking about RAID-1). Am I mistaken in this
thinking?

> > I have no idea how it does
> > this without corruption of some type (as you mentioned above, doing dd
on
> > a live HD will probably cause errors, especially if the live HD is in
> > use). For example, http://www.arcoide.com/ . To quote the function
we're
> > looking at " the DupliDisk2 automatically switches to the remaining
drive
>
> So setup three disks in a software RAID-1 configuration with one disk
being
> marked as a "spare" disk.  Then have a script run from a cron job every
day
> which marks the first disk as failed, this will cause the spare disk to
be
> added to the RAID set and have the data copied to it.  After setting one
disk
> as failed the script can then add it back to the RAID as a spare disk.
>
> This means that apart from the RAID copying time (at least 20 minutes on
an
> idle array - longer on a busy array) you will always have two live
active
> copies of your data.  Before your script runs you'll also have an old
> snapshot of your data which can be used to recover from operator error.
>
> This will do everything that the arcoide product appears to do.

>From what you have said, basically the only advantage to the Arcoide
products are that they reduce load on the system, as they can perform the
RAID-1 mirror process in the background idependent of the OS.

An alternative spin on what you have said (nearly identical) would be to
put double the hard disks in each server (eg. a server has 2 hds, put in 2
"backup" hds). Configure them in RAID-1 mode, marking the 2 backups as a
spare, and then "adding" them to the RAID array every day via cron. This
would cause the 2 live HDs to be mirrored to the backups, and then
disengage the 2 "backup" HDs so they aren't constantly synced.

Would the above work? Sorry if I seem naive, but I haven't tried this
"once in a while" RAID method before.

Sincerely,
Jason