Re: [dm-devel] [PATCH 0/6] dax poison recovery with RWF_RECOVERY_DATA flag

[Dan Williams]

On Thu, Nov 4, 2021 at 10:43 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> On Thu, Nov 04, 2021 at 09:24:15AM -0700, Dan Williams wrote:
> > No, the big difference with every other modern storage device is
> > access to byte-addressable storage. Storage devices get to "cheat"
> > with guaranteed minimum 512-byte accesses. So you can arrange for
> > writes to always be large enough to scrub the ECC bits along with the
> > data. For PMEM and byte-granularity DAX accesses the "sector size" is
> > a cacheline and it needed a new CPU instruction before software could
> > atomically update data + ECC. Otherwise, with sub-cacheline accesses,
> > a RMW cycle can't always be avoided. Such a cycle pulls poison from
> > the device on the read and pushes it back out to the media on the
> > cacheline writeback.
>
> Indeed.  The fake byte addressability is indeed the problem, and the
> fix is to not do that, at least on the second attempt.

Fair enough.

> > I don't understand what overprovisioning has to do with better error
> > management? No other storage device has seen fit to be as transparent
> > with communicating the error list and offering ways to proactively
> > scrub it. Dave and Darrick rightly saw this and said "hey, the FS
> > could do a much better job for the user if it knew about this error
> > list". So I don't get what this argument about spare blocks has to do
> > with what XFS wants? I.e. an rmap facility to communicate files that
> > have been clobbered by cosmic rays and other calamities.
>
> Well, the answer for other interfaces (at least at the gold plated
> cost option) is so strong internal CRCs that user visible bits clobbered
> by cosmic rays don't realisticly happen.  But it is a problem with the
> cheaper ones, and at least SCSI and NVMe offer the error list through
> the Get LBA status command (and I bet ATA too, but I haven't looked into
> that).  Oddly enough there has never been much interested from the
> fs community for those.

It seems the entanglement with 'struct page', error handling, and
reflink made people take notice. Hopefully someone could follow the
same plumbing we're doing for pmem to offer error-rmap help for NVME
badblocks.