Les Mikesell wrote at about 00:16:32 -0500 on Sunday, April 10, 2011:
 > On 4/9/11 10:39 PM, Jeffrey J. Kosowsky wrote:
 > > Les Mikesell wrote at about 11:31:28 -0500 on Saturday, April 9, 2011:
 > >   >  On 4/9/11 12:28 AM, Saturn2888 wrote:
 > >   >  >  :: mdadm ::
 > >   >  This is what I use, with a raid1 created with 3 members but one 
 > > missing.  I
 > >   >  periodically rotate disks into a hotswap sata bay, add it long enough 
 > > to
 > >   >  re-sync, and then fail and remove it to take offsite.  The sync
 > >   >  operation do
 > >
 > > I tried this just *once* and ended up losing 2 years of backups!
 > > Specifically, I had been running software RAID1 on 2 1TB disks for 2
 > > years without any problems. Then on Black Friday, I bought a new 1TB
 > > disk and had the brilliant idea of adding it temporarily to my RAID
 > > array to create a backup.  I mounted it via an external USB
 > > enclosure. Well, somehow it crashed during rebuilding, leaving the new
 > > 3rd disk unreadable and corrupting the pool on the original disk --
 > > specifically, a number of pool files and directories were deleted or
 > > became unreadable. Not sure how and/or why this happened, but the
 > > LESSON is that RAID is not perfect and it can corrupt everything. If I
 > > do this again in the future, I probably should at a minimum remove one
 > > of the internal drives to use as backup later on in case something
 > > happens. Alternatively, I probably would have been better just taking
 > > BackupPC offline and doing a 'dd' to the new disk
 > 
 > I've never heard of raid sync affecting the original disk(s).  I've
 > been doing it for years, first with a set of firewire external
 > drives (which also had USB but it was slower), then the sata bays.
 > There might be problems in adding more members than originally
 > created in the set, though.

I believe that there may be a hardware problem with the USB box I used
to mount the 3rd drive. It actually completed the sync initially (and
I unfortunately then left it connected for a few days). I imagine that
the hardware might have gone flakey during ongoing runs of BackupPC
perhaps leading to the partial corruption of the 2 original drives and the
total corruption of the filesystem on the 3rd drive. 

I also did increase the number of drives in the set from 2 to 3 but I
wouldn't have thought that that would be a problem.


>  In your situation I would have failed/removed one of the internal
> members with mdadm, then added the usb in its place.  With my
> rotation, I make it a point to never have all the disks in one place
> or connected at the same time, so even if something did wipe all
> three connected disks, I'd be able to recover back to the last swap
> from the offsite copy.

I agree that would be a safer approach. I was "lazy" and wanted to
avoid having to re-sync the disk I was removing after the new drive
synced. (I didn't want to use a 3 drive rotation since the original 2
drives are an exactly matched pair so I preferred to keep those as the basis).


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