[ale] Mirroring drives without RAID

[Phil Turmel]

On 02/12/2015 04:42 PM, Alex Carver wrote:
> Got an unusual question from someone at work.  He wants to set up a pair
> of drives to store test data which is usually of significant volume
> (several tens of GBs per test).
> 
> He doesn't want to use RAID so that any single drive can be plugged into
> any other computer to mount and recover the data directly.
> 
> The first thought was just a daily cron job rsyncing the two drives but
> I wanted to ask if there was some kind of solution that faked a mirrored
> RAID onto two drives where each drive is just a standard
> partition/volume set (i.e. it's composed only of ext2/3/4 partitions and
> not a RAID partition/volume)?

Linux raid has a legacy mirror mode that uses entire raw devices and
writes no metadata.  Must be script or manually activated  with the
mdadm --build option.

Or you can use v1.0 metadata that sits at the end of a participating
device.  Then the member devices of a mirror can be accessed without
using the raid.  You have to understand that if you allow any writes you
lose sync.  And most filesystems write to the device when mounting, even
with readonly mounts.

You can also do this with a manually configured device-mapper mirror
(dmsetup) with similar functionality as the --build mode.

Have fun.

Phil