Re: [PATCH 2/2] scsi: scsi_debug: Validate zone access for WRITE ATOMIC (16)

[Niklas Cassel <cassel@kernel.org>]

Hello Christoph,

On Fri, Sep 18, 2026 at 09:06:41AM +0200, Christoph Hellwig wrote:
> On Fri, Sep 18, 2026 at 01:15:36PM +0700, Damien Le Moal wrote:
> > Sure, but it may not be that simple in practice because achieving atomic writes
> > on HDD is not that simple: even though most modern HDDs do have some form of
> > atomicity for single sector writes (e.g. on EPO events), even that is not
> > guaranteed at all. So let's not assume anything that may end up being different
> > than what a real implementation may do.
> 
> Besides that the whole concept of atomic writes on sequential write
> required zones does not make much sense.
> 
> Atomic writes are about atomic updates of multiple sectors, but
> sequential write required zoned never update existing data.  So the best
> they could provide is to guarantee that either all or nothing of a single
> command is appended at the write pointer.  It is very hard to find a way
> to use this feature, as zoned writes all require metadata updates to
> point to the current location, and without this the data won't be
> reached.  There are some schemes to optimizes this by doing a zoned write
> with a header containing the location as a sort of distributed log (zenfs
> in userspace would be the canonical example), but even with that a torn
> write would invalidate the recovery of this header.
> 
> In other words, there really is no point in supporting atomic writes on
> ZBC.  So we should not implement it in scsi_debug, and disable the
> feature in sd.  If we ever see real hardware and a real use case we can
> reconsider, but I doubt it is going to happen.

I have just sent out a new version of this series, and in this patch:
https://lore.kernel.org/linux-scsi/20260918062910.1709791-14-cassel@kernel.org/

a combination of ZBC and atomic writes is rejected by scsi_debug.


This series does not touch sd.c, so changes to sd.c should be a separate patch.
From you reply, it is obvious that you would be able to write a sd.c patch with
a better motivation than anything that I would attempt to cook up.


Kind regards,
Niklas