Adam Goryachev wrote at about 16:08:56 +1000 on Wednesday, April 27, 2011:
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> On 27/04/11 15:44, Jeffrey J. Kosowsky wrote:
> > Les Mikesell wrote at about 12:08:22 -0500 on Tuesday, April 26,
> > 2011:
> >> On 4/26/2011 11:38 AM, Michael Conner wrote:
> >>> However, another idea intrigued me that I saw in an earlier
> >>> posting. Someone used a RAID 1 setup but only put in the second
> >>> disk periodically, then removed it for offsite storage. I have
> >>> three 2T drives, so was considering something similar where I
> >>> would keep a normal 2-disk RAID 1 setup but periodically remove
> >>> one disk and replace it with a prior offsite disk.
> >> It is working for me, but I use a 3-member RAID1 where 2 are always
> >> connected and the 3rd is rotated out periodically. This isn't
> >> really necessary but when I was first trying it with one internal,
> >> one external drive the internal one failed, corrupting the attached
> >> external, and it was something of a hassle to rebuild from the
> >> remaining offsite external.
> > I did it that way where the 3rd 'backup' drive was mounted via USB
> > and had a *catastrophic* failure where something went wrong with the
> > 3rd drive causing all three RAID1 members to become corrupted. I'm
> > not sure exactly what but I ended up losing 2 years of backups.
>
> I suppose everyone has had their own experience with all these things,
> but at the end of the day, this is a pure risk/benefit analysis
>
> > I think a safer alternative would be to do what the OP proposes --
> > that way you always have one safe copy not part of the RAID in case
> > something messes up....
>
> In other words, you are increasing the risk of your one remaining HDD to
> fail during the time that you are syncing the latest removable drive you
> just plugged in. You are potentially decreasing your risk of corrupting
> your RAID array due to some unknown hardware issue with a previously
> used USB HDD.
>
But you still have a removed spare that was up to date as of the time
you removed it just prior to inserting your backup drive. So, at most
you lost a few hours of a concurrently running backuppc process and if
you halted backuppc prior to synching (as per Les's approach -- which
is a good idea to prevent disk thrashing anyway) then you haven't lost
anything, assuming you have nothing else running on that disk.
> As opposed to decreasing your risk to a single disk failure during a
> RAID1 resync (when adding the third disk), and increasing the risk of
> array corruption due to some weird code problem that you may have
> experienced some time in the past...
>
> Not to minimise the issues you had, just wanted to remind everyone to
> properly analyse the risks with the different options they select.
I still think that losing all 3 (which however unlikely is still
possible) is way, way, way, worse than potentially losing 1-2 out of 3
and still having a spare to recover (carefully) from. And my case can
occur if you lose a disk controller or if there is a transient or if
you do something stupid and overwrite the disk, etc...
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