[Gllug] Is this a silly (RAID) notion?

[Andy Farnsworth]

John Winters wrote:
> I have 2 hard discs which I intend to use in a RAID1 array, plus a 3rd 
> identical one to use as a spare.
>
> I notice that the etch installation process offers to set up both active 
> and spare members of the array.
>
> Would it be silly to put all 3 in to begin with, specifying the 3rd as a 
> spare, then once the installation is complete remove the 3rd drive and 
> put it away somewhere safe.
>
> That way, as and when one of the drives fails the 3rd one is already 
> configured as a tentative member of the array and it should be just a 
> case of installing it and activating it.
>
> I don't want to leave the third one in because then it will have been 
> spinning just as long as the other two.  Less head movement of course.
>
> Have I missed something obvious?
>
> TIA,
> John
>   
Yes, this is a silly thing to do.  The RAID setup will expect that spare 
drive to be there every time you boot and will complain if it isn't.  
Rebuilding a two disk RAID 1 array is basically a full copy from the 
single disk to the mirror drive.  If you have a spare drive in the 
setup, one of two things happen and I am not sure which.  I suspect that 
it leaves the drive spun down and only if one of the active drives fails 
does it spin the drive up and begin the full update process on it.  
While the update is taking place, you have no redundancy.  The second 
option is that the drive stays active and is mirrored to (i.e. Drive 0 
is mirrored onto Drive 1 and onto Drive 2 at the same time).  This is 
probably not what happens as it would not be labeled as a spare drive 
but as an active drive.  The advantage of a "Hot Spare" is that if one 
drive fails 1 minute after you have left the office (or house, or the 
server is located physically distant from you) then the hot spare is 
activated automatically and is updated immediately giving you the 
minimum "No Redundancy" time possible.  This is a good thing and as long 
as the drive is not spinning then it is generally not using much in the 
way of electricity and/or creating much heat.

Andy
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