Re: [RFC 1/2] MD: raid5 trim support

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On Tue, 17 Apr 2012 07:46:03 -0700 Dan Williams <dan.j.williams@xxxxxxxxx>
wrote:

> On Tue, Apr 17, 2012 at 1:35 AM, Shaohua Li <shli@xxxxxxxxxx> wrote:
> > Discard for raid4/5/6 has limitation. If discard request size is small, we do
> > discard for one disk, but we need calculate parity and write parity disk.  To
> > correctly calculate parity, zero_after_discard must be guaranteed.
> 
> I'm wondering if we could use the new bad blocks facility to mark
> discarded ranges so we don't necessarily need determinate data after
> discard.
> 
> ...but I have not looked into it beyond that.
> 
> --
> Dan

No.

The bad blocks framework can only store a limited number of bad ranges - 512
in the current implementation.
That would not be an acceptable restriction for discarded ranges.

You would need a bitmap of some sort if you wanted to record discarded
regions.

http://neil.brown.name/blog/20110216044002#5

NeilBrown

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