raid1 mirror optimizations

hi guys... i have a damaged disk...
i=B4m 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=B4t 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=B4s a small loss of cpu/memory)
thanks

--=20
Roberto Spadim
Spadim Technology / SPAEmpresarial
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Re: raid1 mirror optimizations

Hi,

On 2011-01-25 19:56, Roberto Spadim wrote:
> could we implement a 'inverted' mirror? just for hard disks (for ssd
> it´s a small loss of cpu/memory)

If I understood that correctly, this is exactly what the raid10 'far'
layout does.

Regards,
Oliver

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Re: raid1 mirror optimizations

On 25/01/2011 19:56, Roberto Spadim wrote:
> hi guys... i have a damaged disk...
> i=B4m using raid1
> the computer crashed with the floor :P hihiih sorry, but the disks ar=
e
> 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=B4t crash the disk at the sa=
me
> 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=B4s a small loss of cpu/memory)
> thanks
>

If you are worried about the disks being in the same position, then I=20
assume you mean the heads were in the same position when they crashed=20
into the disk. If that's the case, then it doesn't really matter too=20
much if the same bytes on the disk were hit - your disks are trashed=20
anyway, and you'll need expensive professional recovery services to dea=
l=20
with it.

If you are not talking about head crashes, and merely about corruption=20
because the disks were being written to in the same place on both disks=
,=20
then the layout on the disk will make little difference - the same data=
=20
will be written to the same logical place at roughly the same time. It=
=20
doesn't matter where this data is located physically on the disk, since=
=20
it is the data that matters. The same thing actually applies to head=20
crashes too.

If you really want an "inverted" mirror, there is an easy way to get=20
much of the same effect. Instead of setting up raid1, use raid10 with=20
"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=20
each disk. It's not a full reversal, but you can think of disk 2 as=20
being split in two and its two halves swapped.


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Re: raid1 mirror optimizations

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

2011/1/26 David Brown :
> On 25/01/2011 19:56, Roberto Spadim wrote:
>>
>> hi guys... i have a damaged disk...
>> iæƒ=B4 using raid1
>> the computer crashed with the floor :P hihiih sorry, but the disks a=
re
>> 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 donæ„=92 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
>> itæ„=80 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 t=
he
> disk.  If that's the case, then it doesn't really matter too muc=
h 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 corruptio=
n
> because the disks were being written to in the same place on both dis=
ks,
> then the layout on the disk will make little difference - the same da=
ta 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 cras=
hes 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.
>
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-raid"=
in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.ht=
ml
>



--=20
Roberto Spadim
Spadim Technology / SPAEmpresarial
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Re: raid1 mirror optimizations

i don't know what layout does, but, does raid1 have layout?

2011/1/25 Oliver Brakmann :
> Hi,
>
> On 2011-01-25 19:56, Roberto Spadim wrote:
>> could we implement a 'inverted' mirror? just for hard disks (for ssd
>> it=B4s a small loss of cpu/memory)
>
> If I understood that correctly, this is exactly what the raid10 'far'
> layout does.
>
> Regards,
> Oliver
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-raid"=
in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at =A0http://vger.kernel.org/majordomo-info.html
>



--=20
Roberto Spadim
Spadim Technology / SPAEmpresarial
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Re: raid1 mirror optimizations

On 2011-01-26 16:39, Roberto Spadim wrote:
> i don't know what layout does, but, does raid1 have layout?

No, only raid10 has different layouts. For an explanation, see David's
mail, or take a look at the Wikipedia entry for non-standard RAID
levels, it illustrates the idea of the different layouts quite well,
IMO: http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_ MD_RAID_10


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Re: raid1 mirror optimizations

On 26/01/11 17:37, Oliver Brakmann wrote:
> On 2011-01-26 16:39, Roberto Spadim wrote:
>> i don't know what layout does, but, does raid1 have layout?
>
> No, only raid10 has different layouts. For an explanation, see David's
> mail, or take a look at the Wikipedia entry for non-standard RAID
> levels, it illustrates the idea of the different layouts quite well,
> IMO: http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_ MD_RAID_10
>

There are also different layouts for raid5 and raid6, though I don't
think they make much difference unless you are doing something odd like
trying to make your mdadm raid5/6 layout exactly match that of some
hardware raid controller. There are also special layouts that are used
as intermediary steps in converting between raid5 and raid6 - you won't
want to touch these.

So raid10 is the only level with /useful/ layout options. The different
options can make a measurable speed difference, with the best choice
depending on the load. The "far 2" layout is, I think, the best choice
for common ready-heavy loads.


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