[dm-devel] [PATCH] dm-raid: add RAID discard support

[NeilBrown]

On Tue, 23 Sep 2014 19:07:35 -0400 "Martin K. Petersen"
<martin.petersen at oracle.com> wrote:

> >>>>> "Heinz" == heinzm  <heinzm at redhat.com> writes:
> 
> Heinz> In case of RAID levels 4,5 and 6 we have to check for the
> Heinz> capability to zero data on discards to avoid stripe data
> Heinz> corruption.
> 
> I'm quite concerned about relying on discard_zeroes_data since it's a
> hint and not a hard guarantee.
> 

If it is a "hint", then why isn't it "discard_sometimes_zeroes_data"?

md/raid5 already depends on this.  If a read from a discarded region doesn't
reliably return zeros, then raid5 cannot support discard.
Should I turn of discard support in raid5?

Also, could you help my understand when such a hint-but-no-guarantee would be
useful?

Thanks,
NeilBrown
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