raid5.c internal stripe layout

raid5.c internal stripe layout

[Markus Stockhausen]

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Hi,

I'm wondering if there are any pros/cons for handling stripes in memory
exactly in the same layout as on disks. There are several places that could
be simplified if parity disks would be always at the end (DDD..DDPQ). Of 
course one would have to do the mapping during read/write operations.

Cases for optimization would be:

- replace conditions "i == pd_idx || i == qd_qdx" with "i>=pd_idx"
- run data disk loops starting/ending with pd_idx (triple parity raid someday?)
- remove syndrome disk order calculation functions
- Shaohua's last patch could cross chunks
- ...

Just in case I missed a discussion I'm interested if patches are welcome.

Markus
=

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Re: raid5.c internal stripe layout

[NeilBrown]

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On Mon, 18 Aug 2014 18:32:57 +0000 Markus Stockhausen
<stockhausen@collogia.de> wrote:

> Hi,
> 
> I'm wondering if there are any pros/cons for handling stripes in memory
> exactly in the same layout as on disks. There are several places that could
> be simplified if parity disks would be always at the end (DDD..DDPQ). Of 
> course one would have to do the mapping during read/write operations.
> 
> Cases for optimization would be:
> 
> - replace conditions "i == pd_idx || i == qd_qdx" with "i>=pd_idx"
> - run data disk loops starting/ending with pd_idx (triple parity raid someday?)
> - remove syndrome disk order calculation functions
> - Shaohua's last patch could cross chunks
> - ...
> 
> Just in case I missed a discussion I'm interested if patches are welcome.

My guess is that you would just end up moving the complexity from one place
to another.
If I'm wrong and the change ends up making a noticeable improvement in code
readability, then I would certainly consider it.  But I'm pretty sure it
would be a big change, so it would need to bring a real improvement.

So I'm sceptical but happy to be proven wrong.

NeilBrown


> 
> Markus


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