Re: [PATCH 1/4] generic: require discard zero behavior for dmlogwrites on XFS

[Christoph Hellwig <hch@infradead.org>]

On Thu, Aug 27, 2020 at 11:57:03AM -0400, Josef Bacik wrote:
> > 
> 
> This sort of brings up a good point, the whole point of DISCARD support in
> log-writes was to expose problems where we may have been discarding real
> data we cared about, hence adding the forced zero'ing stuff for devices that
> didn't support discard.  But that made the incorrect assumption that a drive
> with actual discard support would actually return 0's for discarded data.
> That assumption was based on hardware that did actually do that, but now we
> live in the brave new world of significantly shittier drives.  Does dm-thinp
> reliably unmap the ranges we discard, and thus give us this zero'ing
> behavior?  Because we might as well just use that for everything so
> log-writes doesn't have to resort to pwrite()'ing zeros everywhere.  Thanks,

So the right fix is to replace issuing REQ_OP_DISCARD bios with
REQ_OP_WRITE_ZEROES (plus the check for that).  That'll give you the
effective zeroing part if the device supports it, and you can still
fall back to manual zeroing (and blk-lib.c actually has helpers for
that as well).