Re: Replacement RAID hard drives - do they have to be "clean"?
On Tue, 16 Sep 2014 15:03:41 -0400
Ken Heard <kenslists@teksavvy.com> wrote:
> My first question is: although both drives are the same size, can I get
> away with having one drive a Seagate 3.0 and the other Samsung 2.0?
Indeed, this is a very recommended configuration, as HDz of the same
brand (and much worse: same brans/model/series) have many more chances
to break down together.
> It occurred to me that if I made the change described in the first
> paragraph -- but without somehow making the data already on it
> unreadable -- there would be a different data set on each drive; so
> that the RAID1 software would not necessarily know which drive should
> be the data source to copy to the other drive. It also occurred to me
> that the software could combine the data on each drive, so that both
> drives would have both data sets.
That doesn't work like that; first, you have a special block that tells
who's who, 2nd, as you'll add the samsung to the array, the system
considers that last arrived = new = slave, not master (of data).
> I consequently assume that the data on the replacement drive must
> somehow be made unreadable. Is that assumption correct? If so, do the
> data have to be "shredded", or is it sufficient simply either to
> "delete" them or simply reformat the drive?
That doesn't matter as the software RAID will detect data presence (IF
superblocks are at the same place, read: same partition(s) as now), thus,
it MAY complain about this data presence and you MIGHT be obliged to force
data overwrite on it.
> Finally, once I have a "clean" new drive installed, will the RAID1
> copying process partition the new drive the same way as the other drive
> and copy the files without further human intervention?
Keep in mind that it can fail for the following reason: HDz aren't
exactly the same size (except when same brand, model, series); so
if your samsung (partition or full HD, YMMV about raw RAID or not)
is even 1 sector less than the ST one, RAID will refuse to reconstruct.
More isn't a problem as RAID will only use what it needs.
If you reach this point, I strongly suggest making a bit copy (dd) of
the working ST HD, then try to shrink it (if the FS allows it, eg:
XFS doesn't), then if it succeed, re-add the samsung.
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