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

[Christoph Hellwig <hch@infradead.org>]

On Tue, Sep 23, 2014 at 07:07:35PM -0400, Martin K. Petersen wrote:
> >>>>> "Heinz" == heinzm  <heinzm@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 we want to provide useful discards through raid we need to make
it a hard guarantee.  That is:

 - only set it when using a WRITE SAME variant on scsi
 - start adding a whitelist for ATA

Relying on zeroout which also works when it doesn't unmap below is not
the answer, especially if it may end up doing a physical write of zeroes
on disk devices.  Alternatively we could get rid of discard_zeroes_data
and stop the pretence of any sane behavior from discard, and instead
expose an unmap bit through the blkdev_issue_zeroout stack for those
callers who want to do a discard, but rely on zeroes.  The end effect
will be the same, just a different presentation.