Re: raid1 mirror optimizations

[Roberto Spadim <roberto@spadim.com.br>]

nice :) i never read about it on raid 10, maybe i could use, thanks!

2011/1/26 David Brown <david@westcontrol.com>:
> On 25/01/2011 19:56, Roberto Spadim wrote:
>>
>> hi guys... i have a damaged disk...
>> i惴 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愒 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愀 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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-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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