Re: raid1 mirror optimizations

[David Brown <david@westcontrol.com>]

On 25/01/2011 19:56, Roberto Spadim wrote:
> hi guys... i have a damaged disk...
> i´m using raid1
> the computer crashed with the floor :P hihiih sorry, but the disks are
> damaged at the same position
> check: http://www.spadim.com.br/hd%20agra.zip
> the problem: since raid1 (mirror) is done with real mirror, the disk
> position are the same...
> if i was using a mirror but on disk 1 i write from beggining to end,
> and disk 2 from end to beggining , i don´t crash the disk at the same
> position, for disk 1 i crash it some bytes, for disk 2 i crash some
> others bytes, since beggining is a small cilinder and end a bigger, i
> could loose less information than mirror
> could we implement a 'inverted' mirror? just for hard disks (for ssd
> it´s a small loss of cpu/memory)
> thanks
>

If you are worried about the disks being in the same position, then I 
assume you mean the heads were in the same position when they crashed 
into the disk.  If that's the case, then it doesn't really matter too 
much if the same bytes on the disk were hit - your disks are trashed 
anyway, and you'll need expensive professional recovery services to deal 
with it.

If you are not talking about head crashes, and merely about corruption 
because the disks were being written to in the same place on both disks, 
then the layout on the disk will make little difference - the same data 
will be written to the same logical place at roughly the same time.  It 
doesn't matter where this data is located physically on the disk, since 
it is the data that matters.  The same thing actually applies to head 
crashes too.

If you really want an "inverted" mirror, there is an easy way to get 
much of the same effect.  Instead of setting up raid1, use raid10 with 
"far 2" positioning.  The effect is roughly like this:

disk1 (stripe 1) (mirror of stripe 2)
disk2 (stripe 2) (mirror of stripe 1)

So the two copies of the data are in different physical positions on 
each disk.  It's not a full reversal, but you can think of disk 2 as 
being split in two and its two halves swapped.


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