Re: raid0 hashing funtion

[Andre Noll <maan@systemlinux.org>]

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On 08:12, Neil Brown wrote:

> On reflection, I am not convinced that the hash really does add value.
> You cannot have more entries in the list than there are drives, and
> you usually have much fewer.  Given that consecutive entries in the
> list are likely to be in the same cache line, and the hash_table is in
> a totally different page, probing 3 or 4 locations in the list is
> probably faster than doing a lookup in the hash table.  And as we
> would be able to get rid of that sector_div, we would probably get
> even more saving.

Yup, that's exactly what I was thinking when I looked at the code.

> The only cost would be if someone had a raid0 with lots of devices all
> of very different sizes.  I suspect that case does not need to be
> optimised for.  So I would probably be happy to accept a patch which
> removed that hash table.

I'll see if I can come up with a patch that gets rid of the hash table
and that one sector_div. Killing all instances of sector_div in raid0,
while possible and probably desirable, would be more involved though.

Thanks
Andre
-- 
The only person who always got his work done by Friday was Robinson Crusoe

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